Battery Back-up problem

I have three servers each with its own APC Battery back up. server 1 has a smart UPS 1400. server 2 has a smart ups 1500. server 3 has a smart ups 1500.
The problem We installed a back up generator to power our computer room, but when we were testing it servers 2 and 3 back ups kept going on and off of battery. Server 1 had no problems. They are all plugged into the same lines.
The generator was installed by a qualified electrician. What could the problem be?