The 6WD Wild thumper has DC motors with a stall current of 5.5A, as the TREX controller has two channels, I’m going to assume you have 3 motors connected in parallel, so the approx stall current would be 16,5A but the controller is rated 18A so that shouldn’t be the problem. The other thing that could be causing issues is the battery’s discharge rate, if you’re not familiar with this I suggest checking this guide:
The continuous discharge rate of a battery is very important because if you choose a battery that cannot discharge at the required current, the robot will either not work properly or not work at all.
You selected four 12V motors for your 4WD outdoor mobile robot. Each motor consumes 1A under normal load, and more in the case of a slope. You decide to choose a 12V, 2Ah NiMh battery pack, not caring about the continuous discharge rate. You discover that your robot stops when it encounters even the slightest obstacle or incline. Why? In this case operating all four motors consumes ~4A while an NiMh pack can only discharge at about 1.2 times the capcity (1.2 x 2Ah = 2.4A). The current draw from the motors is therefore higher than the battery can provide.