ActiveSync don't work correctly for iphone mobile device

Hi, we have deployed last year an iapp exchange2010 that works well for all : owa, rpc, outlook anywhre, autodiscover and ActiveSync. now we have experiencing an issue with mobile device like iphone that is disconnected from ActiveSync somethimes without reason. (i see also that load balacing just for ActiveSync not works fine : 2 servers from 4 that don't receive match traffic like the other)

thank you for your help.

F5 version 11.4.1 HF9

see bellow the configuration :

ltm pool /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_as_pool { app-service /Common/EXchange_CAS.app/EXchange_CAS load-balancing-mode least-connections-member members { /Common/172.21.151.32:443 { address 172.21.151.32 } /Common/172.21.151.33:443 { address 172.21.151.33 app-service /Common/EXchange_CAS.app/EXchange_CAS } /Common/172.21.151.34:443 { address 172.21.151.34 } /Common/172.21.151.35:443 { address 172.21.151.35 } } monitor /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_https_monitor } ltm rule /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_persist_iRule { app-service /Common/EXchange_CAS.app/EXchange_CAS when HTTP_REQUEST { switch -glob [HTTP::path] {

    "/Microsoft-Server-ActiveSync*" { 
                  Direct all ActiveSync clients to a common pool; use 
                  HTTP cookie persistence 
                 persist cookie 
                  pool EXchange_CAS_combined_vs_as_pool 
             } 
    "/rpc/rpcproxy.dll" { 
                  Grab all requests for Outlook Anywhere; the following 
                  checks assign correct persistence methods. 
                 switch -glob [HTTP::header "User-Agent"] { 
            "MSRPC" { 
                          This User-Agent section matches most versions of 
                          Outlook and Windows using Outlook Anywhere. 
                          The OutlookSession cookie is new to Outlook 2010. 
                         if { [HTTP::cookie exists "OutlookSession"] } { 
                             persist uie [HTTP::cookie "OutlookSession"] 3600 
                         } else { 
                             persist uie [HTTP::header "Authorization"] 3600 
                         } 
                     } 
            "*Microsoft Office*" { 
                          This section matches some versions of 
                          Outlook 2007 on Windows XP 
                         persist uie [HTTP::header "Authorization"] 3600 
                     } 
                     default { 
                          This section catches all other requests for 
                          Outlook Anywhere, and sets a persistence method 
                          that does not require the client to support 
                          HTTP cookies 
                         persist source_addr 
                     } 
                 } 
                  Finally, this assigns the Outlook Anywhere pool and turns 
                  off full HTTP parsing and compression. If the preceding 
                  clients should be sent to separate pools, the pool statement 
                  should be removed here, and a separate pool statement 
                  placed in each of the preceding logic branches. 
                  Other modules (APM, ASM, etc.) should be disabled here 
                  as well, if active for other traffic though this virtual 
                  server. 
                  pool EXchange_CAS_combined_vs_oa_pool 
                 CACHE::disable 
                 HTTP::disable 
                  COMPRESS::disable 
             } 
    "/xml/autodiscover.aspx" { 
                  Requests for Autodiscovery information. The selected pool 
                  might be unique, or might be the same as e.g. your pool 
                  for OWA or ActiveSync. In this example, we use the same 
                  pool that receives ActiveSync traffic. 
                 persist cookie 
                  pool EXchange_CAS_combined_vs_ad_pool 
             } 
             default { 
                  This final section takes all traffic that has not 
                  otherwise been accounted for and sends it to the 
                  pool for Outlook Web App 
                 persist cookie 
                  pool EXchange_CAS_combined_vs_owa_pool 
             } 
         } 
     }

} ltm virtual /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_https_virtual { app-service /Common/EXchange_CAS.app/EXchange_CAS destination /Common/172.21.150.100:443 fallback-persistence /Common/EXchange_CAS.app/EXchange_CAS_source_address_persistence_profile ip-protocol tcp mask 255.255.255.255 persist { /Common/EXchange_CAS.app/EXchange_CAS_cookie_persistence_profile { default yes } } profiles { /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_caching_profile { } /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_clientssl { context clientside } /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_http_profile { } /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_oneconnect { } /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_serverssl { context serverside } /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_wan-optimized-compression_profile { } /Common/EXchange_CAS.app/EXchange_CAS_lan-optimized_tcp_profile { context serverside } /Common/EXchange_CAS.app/EXchange_CAS_wan-optimized_tcp_profile { context clientside } /Common/ntlm { } } rules { /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_owa_append_iRule /Common/EXchange_CAS.app/EXchange_CAS_combined_vs_persist_iRule /Common/Exchange_hsts_ltm } source 0.0.0.0/0 source-address-translation { type automap } translate-address enabled translate-port enabled }