I wonder if has something to do with mono vs. stereo - maybe you need to use an 1/8” to stereo 1/4” cable? This is how I record mine.

UGREEN 3.5mm 1/8" TRS to Dual 6.35mm 1/4" TS Mono Stereo Y-Cable Splitter Cord Compatible for iPhone, iPod, Computer Sound Cards, CD Players, Multimedia Speakers and Home Stereo Systems 6FT https://www.amazon.com/dp/B00ZKM3S4S/ref=cm_sw_r_cp_api_i_ZzMwCbVPC6V9C

Out of curiosity, what happens if you turn up the drum group (hold mixer button at the top and turn green dial)?

This one will, but you can also get something like this that breaks it out into left and right mono 1/4 plugs

You use both mono inputs to create stereo. So use a stereo aux to two 1/4 inch jacks, eg: https://www.amazon.com/UGREEN-Splitter-Compatible-Computer-Multimedia/dp/B00ZKM3S4S/=1?th=1

Are you using a "3.5mm stereo to Dual 6.35mm Mono" Cable like this?
https://www.amazon.com/UGREEN-Splitter-Compatible-Computer-Multimedia/dp/B00ZKM3S4S