We recently built a small data center where I work. It has two rows of three racks each, back to back, so as to create a hot aisle between the two rows of servers (at the back of all the racks).
We had spec'd it to have two return air vents over the top of the hot aisle, and then have the cool air being blown in via registers all around the room. However, the HVAC guys thought we were crazy, so they put one large return air vent on the wall (closest to where the outside HVAC unit is) and put a few cold air registers over the server racks.
What seems to be happening is the cold air is simply being blown into the room and sucked out, without proper cooling of the server racks. We are struggling to keep the room cool.
Should we go back to our original plan, which was to have cold air vents circling the room, and the return vents over the top of the hot aisle between the server racks. The theory was this would suck the air 'across' and through the servers', and then the return air vents would im################y pull hot air out of the room.
Your best bet is to engage an engineer familiar with data center cooling. There are a lot of variables to balance. It sounds like your original design has promise, but the contractor didn't build it right. I'd recommend getting a design that is verified to work on paper before asking the contractor to rework the job.
In the area that I live, there is no one familiar with DC construction, so we have a tendency to rely heavily on internet research. I think we had come up with something that would work, but the local HVAC guys ignored it and put in venting that they would do for a normal room.
I might try and find an engineer I can work with long distance to see about the redesign.
As stated before you need to contact someone that work with you specifically looking at your hot air return,your cooling and try and eliminate any commingling of the cold and hot air to name a few.