[dpvu]

AEGs have such a wide variety of tolerances now you really just have to find a brand of mags that work and you like. With the BBs that don't work if when you release the mag no BBs fall out and there's not one in the hop up then the issue is the hop up was not pushing the retention piece back far enough or the mag is not seating high enough. This can be fixed by filing the retention piece on the mag a little sometimes but if you go too far it won't hold BBs at all.

Since you did get it to work with other BBs I would guess the mag seats high enough but the dimensions of the Valkens were just causing them not to be able to feed possibly due to being ever so slightly bigger or small enough that they're binding on each other in the hop.