Hello Subin,
the most likely issue is that the fixed step time is too large to simulate accurately. Variable step solvers automatically make the step size smaller if necessary to try and ensure an accurate simulation. If changing from variable step to fixed step makes your results worse, the most likely culprit is that you need to use a smaller fixed time step for your simulation. Some other issues. If you changed your Simulink solver to fixed step, most of those solvers don't actually work well with simscape solvers. You can go to the simscape solver configuration block and set that to local solver, backward euler, and then the simscape network will also run fixed step.
Regards,
Joel
