My robot is not moving in a straight line. Ros-gazebo

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My robot is not moving in a straight line in ros1. I took the code from some place it dosen't look good. I have checked for the cenre of mass and also joints. I think the problem is with the contact on the ground of the simulation in gazebo. Initially all the 4 caster wheel and 2 diffrentila wheel. when they are placed on the same plane, the robot dosen't move. But when I increase the size of the diffrential wheel, the robot moves but dosent go in a straight line((It almost has like 10 degree curve to right or left). It works fine with PID controller but I need to plot the odometry data, so i need it to be perfectly straight.

Please help, thanks in advance!