My poochyena is at level 15 and it didnt learn bite why?

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1

I think i was explicite enough up in the subject

2

It might be because a pokemon wanted to join your team at the same time as you leveled up.

But don’t worry, you can always relearn Bite at Electivire in town with a Tiny Mushroom.

3

http://www.serebii.net/pokedex-bw/261.shtml says:

Poochyena learns Bite at Level 13.

Either way, like Fokkussu said, find a Tiny Mushroom and relearn it at Electivire in the southeast corner of Grassroot.