To see results like that going from 15 yards to 25 yards I'd say there areseveral possiblities. First---it's you. Even a grossly untuned bow will group arrows. They'll just fly ugly getting there.

2nd-----When you change things like strings, rests, or whatever, however minor, you should retune the bow or at least check out the tune. String and rest type and location are not minor changes. A complete retune is called for. How you do it is up to you. Paper, bare shaft, walkback. They all work. Walkback is better for checking centershot. Paper tells you how the arrow is leaving the bow. Bare shaft is good for both, plus checking arrow spine, but is more critical of consistent shooting form. Once you have it tuned pretty well with target points then try it with broadheads. Use the broadhead tuning section of Easton's Tuning Guide for a good reference.

CONTACT---Is the rest set up correctly? Arrows will scatter if the fletching is hitting the rest. Drop aways are nice but they have to be timed so they get out of the way before the fletching passes by. Try drawing your bow slowly and see. The rest should not raise up till about the 2" of the draw cycle. If it raises too soon it won't drop early enough.