look at the image to configure Sflow , and look at the detail of solorwinds;
Solorwinds detail for node f5 are not avaliable because netflow and CBQos data are not avaliable.
I'm really not sure what's happening with your situation but I did find a couple documents that cover Sflow:
Manual Chapter: Monitoring BIG-IP System Traffic with sFlow - https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-external-monitoring-implementations-11-3-0/9.html?sr=59304758
SOL15193: sFlow traffic will use only the self IP address of the BIG-IP system as the egress interface - https://support.f5.com/kb/en-us/solutions/public/15000/100/sol15193.html?sr=59304758
Manual Chapter: Configuring Performance Monitoring - https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/tmos-implementations-11-2-1/24.html?sr=59304826
Also if you go to support.f5.com and search 'sflow' you'l get some other resources.
hope that helps.