Have you tried using different kits? Some of the drum machines have very similar sounding toms or snare and clap or other things. Usually, when I get stuck it's because 2 similar sounds are playing at the same time and one of them is getting drown out by the other.

I often get stuck at a certain level too. It's tough when you keep getting it wrong for days on end. I also try not to get too fixated on Sound Gym and limit myself to two or three challenges a day.

Even if you think you're doing it correctly, you might find yourself deviating from the reference in unexpected ways. So, when you're getting it wrong repeatedly, how about trying to judge by looking at the sequence light at the top and the timing of when you hear the sound?

As Mr. Jay Key said, changing drum kits makes it easier to distinguish the sounds and helps you understand the character of the kit, so I highly recommend it.