This is a common problem. Your keyboard cannot press J K and L at the same time. Mine can't either, so I use SDF HJK instead. That might not work for you, but you can use the page Cuppp posted to try to find a key setup that does work properly.
Looking for mods for my mania map Minori Chihara - Kyoukai no Kanata (TV Size). Need some tips on how to improve in osu!mania? Try reading our guide and map suggestions! Need some tips on how to map in osu!mania better? Try reading this guide! Looking for the o2jam skin I use? This folder has it and a few other o!m skins! Message me if this link doesn't work, and I will update it.