motors behaviours

Hello SDR,
I got a SDR standard 4-wheeled platform with 4 motors of
“IG42 24VDC 122 RPM Gear Motor with Encoder”
and got Roboteq SDC21xx motor controller to control them.
The motor controller is two channel only.
i control the left two motors in one channel together.
and the right two motors in one channel together.

P.S. the left two motors have different model number than the right two motors.

Also, i got two
IG32, IG42, and IG52 Gear Motor Encoder Pull-up Boards
connected to the front motors to get some encoder values.

Now, the problem is that when i disconnect the pull-up boards (disconnect the encoder), and give a command to go forward a bit and stop, the motors behaves exactly as the command says. which is go forward a bit and stop very accurately.

when i connect the pull-up boards the motors behaves differently, when i give a command to go forward and stop, it goes forward and doesn’t stop. it either goes forward and starts turning in a circle or goes forward and make a slight turn and stops.

i want accurate movement since I’m using encoder values. i don’t want this slight turn to occur, i want it to behave exactly like its behavior without the pull-up boards.

have any idea?
thank you