Since it is multiple bows, I would not suspect the rest, at least not at first.
Assuming a well-designed bow, your inconsistent results are probably tuning and form related.
When you say the arrows are spined correctly, do you mean their given spine matches the weight of the bow? Arrows need to be tuned to the bow, regardless of their given spine. I recommend bare shaft tuning. And remember, spine is relative to arrow length and point weight.
Once you are sure you have arrows matched to the bow, then tune for brace height and nock point.
All this assumes a consistent form, particularly anchor point and release. That comes with experience and practice.