I don't know of any round-robin like NAT pool method. It makes sense that the FortiGate won't use another IP of the pool unless there is a source port conflict or unless there are a lot of connections.
Why do you want to "use" the IPs in a uniformly distributed way ? Is there a real purpose ? If yes and NAT isn't doing what you expect, you could:
- split your network and your IP pools in different rules so that, for ex, IPs from 1-50 use the pool1, 50-100 use the pool2 and so on...
- somehow use the wan link load balancing feature. I don't know if it's feasible on one interface alone, I suppose it isn't. In order to avoid this, you could split the public IP range on your router and have those ranges end up as different interfaces on your firewall. Then you'll be able to use the wan load balancing feature.