Hey guys in the past with my 100cc mx2. And my 50cc yak I had no problem hitting my knife edge spin but since I have got my 104 slick and my 100cc Sbach I can't seem to nail the knife edge spin

You probably have the most common problem with the Slick in knife edge spin and that's too much aileron. The control surfaces are big. Huge! The ailerons especially. Therefore it doesn't need full aileron stick on 3D rates. It usually doesn't quite need full rudder either. Next time you enter the spin, do it a little higher up and as it rotates, ease off some aileron. When you hit the sweet spot, it'll wrap up into a tight, crazy fast spin. If it wants to go straight into a flat spin, you're still using too much aileron. A higher altitude will give you more time to find the right amount of input.

My 50cc Sbach needs next to no aileron in a knife edge spin.

Also, too much expo makes it hard to do flat and knife edge spins becuase you too much deflection for very little stick movement in that 1/2 to 3/4 stick area where you're trying to finesse it.

Less rudder and you'll nail it.