If you've plugged the kick pad into another input and experience the same problem as well as plugging another pad into the kick input and experience no problems I would suggest there is a problem with the kick pad itself. Possibly a bad or failing piezo or perhaps the adhesive that holds it in place has let go and it has shifted off center or one of the wires is not soldered quite right. In either case, it's probably a simple fix by disassembling the kick pad and perhaps replacing or resoldering the piezo.
I've had to fix or replace them on my snare pad at least 3 times since it gets the most abuse as I play. Usually, in my case, it's just a wire has come off of the solder point.
One other thing to check before you disassemble it is have you tried switching a known working cable from the kick pad to the module?
I had an issue once where I thought one of my pads wasn't responding quite right and it turns out the cable had an issue. I swapped it out and it fixed the problem. Also check the physical jack on the kick pad for dirt/debris inside preventing a good contact with the cable jack.
Hope this helps.
Powerhouse