If it is some users then it is probably local issues, not related to the SSLVPN. If it is all users (from one site i find odd, or do you mean one destination SSLVPN?) then there might something on the SSLVPN side which can be changed.
In any case I would involve F5 support. You will probably try some different settings and check on results, perhaps packet captures. F5 support can actually look into your system with you and determine all these things. Doing that via a forum like this gets complicated and possibly impossible to share confidential information.