Load Balance Question

I'm am new to bigip and herited a couple and have a question.

 

I have a pool with four nodes, round robin with priority group activation set at less than one. Ratio is all set at one and the priority group I have a node at 2 and other three nodes at 1. My problem is that sometimes the node with 2 priority gets overloaded and traffic is bounced to the other nodes, but it seems that all the traffic goes to one node with priority 1. Everytime this happens it does the same thing except it may be another node. I thought that it was susposed to round robin on the other three nodes? I don't know why it is not doing this. Any thoughts answers, questions or corrections, please let me know.

 

 

Thanks

 

Frank