Unable to connect multiple VPN users (more than 3)

hello,

we have a fortinet 200d Firmware: v5.2.3,build670 and about 15 vpn users.

We are all using forticlient 5.4 (some use 5.4.1 and others 5.4.2).

One or two users can connect with no issues (IPSEC).

As soon as a third user connects one of the other two users is disconnected and each time only two users can be connected simultaneously. 

We need to allow 4-6 users in simultaneous connection.

Thanks for the help.