The current is set to 1.5A
That is one of your problems. The A4988 can supply only 1 Ampere continuously unless you have good forced air cooling and a finned heat sink. Especially at low speed, it will certainly overheat and shut down at the 1.5A setting -- if that is actually what you have done.
Since different boards have different current sense resistors, the calculation in the link you posted may not be correct. What is the value of the current sense resistor on your A4988 module? Post a clear, close up photo if you do not know how to recognize it.
This is a much better driver: https://www.pololu.com/product/2966