WorkerNodes not Balancing evenly

Hello a have a query, which would be a possible cause for which in a Cluster where I have for example:
3 MasterNodes & 6 WorkerNodes, the load balancing for the processing of the WorkerNodes, is not homogeneous. I refer to for example of the 6 WorkerNodes, 1 WorkerNode has overflowed in RAM while the other 5 WorkerNodes are at 18%, 20% of use. As the ** Round ** - ** Robin ** was not working homogeneously.

On the other hand, what mechanism could be applied here. Thanks.

Some idea my friends ???