hosts get different public IP adresses

Hello,

I have a problem with fg-50E. I configured on forti WAN1 and WAN2.

Most hosts get their public IPs from WAN1 (eg. 212.55.66.77) but few of them from WAN2 (195.55.66.77) 

Do you know how to change it, and force that all hosts get their IPs from WAN1?