There are several things that can cause this on that assignment: omitting the stride in the computations for either conv_forward or pool_forward will pass the tests in the notebook, but fail the grader. The stride logic should be the same in both. Since you got 75/100, at least one of those functions must be correct. Coercing the output type of conv_single_step to float32 will also cause a loss of points.
I don’t know why the grader here can’t at least tell you which functions failed. We’ve complained about this for a long time to the course staff, but apparently this limitation of the grader is hard for them to fix.