It has to be either a lack of ‘oomph’ from the motors or too much friction. Having had a go at the ‘too much friction’ side of things lets have a go at ‘not enough oomph’.
When you say you have adjusted the Vref correctly what value have you set them to?
I set my DRV8825 drivers at 1.1V Vref. But a word of warning here, I force cool the stepper drivers with two 40mm flushing fans. A genuine polopu DRV8825 utilises a 0.1ohm sensing resistor and has a working formula of -
Current Limit = VREF × 2
so a Vref of 1.1v for me represents a coil current of 2.2Amps…the maximum for the motors. Stuff is going to get hot! I keep a check on the motor temperature during a job run. I have not yet had a long enough job that caused me problems but it could be an issue in the future. Additional 40x40 heatsinks are available for the bottom/side of the motors to mitigate some of the heat.
To check you have the correct value sensing resistor on your drivers check the markings on your boards. The sensing resistors (there are actually two of them) are located next to the trimmer pot on the pcb, they are labelled R100.
Assuming yours are 0.1ohm sensing resistors and assuming you have the correct stepper motors (that Ryan supplies…their dimensions should be 42x42x52) they have a max current rating of 2.2Amps.
Ah…re-reading your earlier posts I note you have A4988 drivers so forget all that then…