Why different pumping rates may cause the model not to converge？

[YANG-814]

 I am currently experiencing the following problem when carrying out a numerical groundwater simulation using flopy:
 I have 10 pumping wells set up inside my model with a total of 31 pressure cycles. When I set the pumping rate of each well to -50, the programme works fine. But when set to -37, the program does not execute properly and displays the error:" FAILED TO MEET SOLVER CONVERGENCE CRITERIA".  Why is this?
 Thanks in advance for your answers.