Unreachable VPN

Hi guys,

I have a problem regarding the SSL VPN. the team use forticlient 6.0.3 but sometimes it shows unable to connect then after a while it works. so i have checked the version of the 100D that i have and it is using v5.6.4,build1575,180425 (GA). the documents says it might be compatibility issue, but the Fortinet compatibility chart shows it is compatible.

any suggestions