I was able to find the culprit. I think it is a log issue and my memory is almost full. My settings on all my policy is enable logs "all sessions". But based on what I know with Fortinet this should not have happened since by default fortinet will refresh the memory once it reaches 70%. This I am not sure. Correct me if I am wrong. I'll just type here the error.
app-ctrl memory log is 95% full
app-ctrl memory log is 90% full
app-ctrl memry log is 75% full
I think based on the logs Fortinet is trying to clear memory. BUt the problem again I am not able to access the GUI thru web interface. Since I need to do live monitoring of my network. so I tried to restart my fortinet but it was taking a long time to boot, almost an hour. So I tried to force shutdown which is oncorrect but I have no choice since I cannot access the Firewall GUI. I did this by removing the power cord. After turning on again the power after 5min. it took my fortinet around 15min to boot and now I can see my GUI. After I logged in and change the log settings on all policy to log only "Security Events Only". After that until now my firewalls are running stable.
Now my question since my setup is HA why my second firewall did not kick in and do the work while I restart firewall A. Is this possible with Fortinet?