Hi there,
Andre hit the nail on the head when he said that DRS is not a true load balancer.
It will only apply rules if it deems that there is a resource contention issue and moves VM's appropriately to improve performance. To be honest I was expecting the same thing the first time I enabled DRS :-)
The only way to help better spread your VM workload would be to create custom rules that either use VM affinity or anti-affinity, this way you could keep VM's together such as an app server and db server that talk to each other, or you might say that a group of heavy IO vm's can't all reside on the same host.
Cheers,
Ben Loveday