I posted this on Raspberry Pi CNC already but doesn't give enough attention and I think it will fit better over here. Sorry for the spam.

I'm building a 3 omniwheels rover platform.

Setup:
- Protoneer 2.6
- Raspberry 3B+
- 3x A4988
- 3x NEMA17
- 3x Omniwheel d = 100mm
- Microstepping: 1/8
- 14.8V supply voltage

I burnt the bootloader onto the shield and upload an Arduino sketch onto that (using AccelStepper to control 3 motors with different speed).

Currently, this setup gave me a very low torque. Sometime, it could not even drive a simple rover platform. The drivers heat up quite a lot though (even with heatsink).
Is there any way to overcome/improve this (the torque)?

More importantly, is this a normal behavior and why? Thanks.