Using Protoneer board with A4988 drivers does not give me enough torque.

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.

what is the spec of your motors, you need to know the current they need and the momentum they can deliver. Aren't you expecting to much, 100 mm wheel is big for such a small motor
Yes the A4998 easy get's hot if you pull more then 1 A to the motor. Know that the motor current is (much) higher than the supply current from your 14.8Volt
A4998 drivers can do max 2A ( with proper cooling ) Increasing the voltage to 24 volt will also give a better performance and lower temperature.
If you just need a bit more current, then the DRV8825 might be a good idea, as it's almost pin compatible

Hi,

I recorded a little video of the rover in action.
Can you give me some suggestion what is happening? I'm seeing step skipping, but then what else?

Thanks.

https://www.youtube.com/watch?v=KnyDPhHAKUI

Put the rover on a block such that the wheels can rotate free ( hardly any torque needed ). Then have a look if the wheels rotate as expected from your program.
If they do, then for sure it's lack of torque. If only one does, have a look at your code

You are right, it is the lack of torque since the wheels can always rotate freely on air.
What should I do to increase more torque for each wheels?

To increase the torque you have a few options. First have a look at the motor controller you have. What is it set ti output for torque? secondly have a look at your motors. What are they rated at for ounce inch specs? Also make sure the wires are not undersized.... going to the motor controllers OR going to the motors. What voltage are you running. Most robots like that you want to be running at least 12v. you could try running 24v to the motor depend on the controller specs.

I'm no expert on Raspberry Pi matters,but I would doubt that you are getting enough electrical power to your steppers.Have you watched any of the youtube videos about using Arduinos to control large steppers?This one shows the Arduino controlling a driver module,rather than directly operating the steppers https://www.youtube.com/watch?v=RakXequOrSY .I do understand that a Pi isn't an Arduino but the principle of using a small,low voltage board to drive a stepper seems unlikely to work as well as using such a board to control a driver module at which larger currents are utilised.