WAN FAILURE

Hi there , 

i'm new to fortinet product and fortigate firewall , the problem is i managed to configure wan load balance with seasion algorithm on 3 WAN interfaces ( switched 1 lan port to wan ) and i've like 3 difference switches for different locations inside the company premises , however when i connect 1 or 2 switches " 16 and 5 ports D-link " the internet is working fine however when i do connect the last " 24 port switch " somehow the whole network crash and no one can access the internet . 

i'm using fortigate 60D .

only 1 lan 1 subnet 

around 50 - 60 user using the internet 

Note :- when the network crash i do switch back to Pfsense firewall and everything works fine