some LTM virtual servers are marked down in BIGIP DNS

We have 1 BIGIP DNS pair and 1 LTM pair. all of them are running on bigip version13.

We have configured 5 LTM virtual servers. All 5 are up in BIGIP LTM. But in BIGIP DNS, 2 of them are marked up and 3 are marked down.

Dec 14 09:13:14 prz-dns01-bigip1 alert gtmd[26431]: 011ae0f2:1: Monitor instance /Common/bigip 10.63.232.210:443 CHECKING --> DOWN from /Common/prz-dns01-bigip2.com (no reply from big3d: timed out) Dec 14 09:13:14 prz-dns01-bigip1 alert gtmd[26431]: 011a6006:1: SNMP_TRAP: VS /Common/vsr-syd04-vra (ip:port=10.63.232.210:443) (Server /Common/prz-lb02) state change blue --> red ( Monitor /Common/bigip from /Common/prz-dns01-bigip2.com : no reply from big3d: timed out)

I checked the traffic group config, looks good.