The flow rate was not strong enough for the rain shower.  I replaced the rain shower head with a smaller diameter version and still could not get an adequate flow.  

I then moved the smaller shower head to our second bathroom and the thing works nicely so I know the issue is with shower system pipes, and not my plumbing.

This unit is rated at 1.8 gpm.  I measured it after it didn't work very good, and that is what I got, so my suggestion is to make sure you check the flow rate.