Users Disconnection Randomly

Hello;

I've been experiencing a new problem on my organization, I have 2 buildings that i connect them B2B, Installed a new Fortigate 200E on buidling B, After few weeks now I have this weird problem, Some users randomly experience internet disconnection, While I try to ping to my domain or any ip on my network it completes successfully, But when I try to ping my Fortigate ip, It doesn't work..

Only pinging this user's IP from Building A solves the problem and the internet works, Or restarting the Fortigate.

I suspected some loop but I didn't connect any new cables or switches for weeks now, And I'm using my domain's DNS Server not the Fortigate's.

So if there's anyone could help me with this problem I'll appreciate it.