Cubase AI - sometimes no sound produced

Hello all,
I have the Steinberg CMC PD controller pad and when I play it, it is recognised by the green vertical vu meter and in some instruments (i.e ‘bells’) it does produce sounds. But when I play it in other certain instruments (i.e ‘rock kit’) no sound is produced yet the vu meter still recognises it. Can anyone please help me?

Thanks in advance.