Sounds to me like your draw weight has decreased given your pins have not moved ..........You stated your horizontal grouping was right down the middle, so this indicates a good form and stance......I once experienced a 5" drop in my verticle grouping but was due to removing the rubber peep tube on my peep sight!!!!! I was trying to eliminate extra stuff and stumbled across this efect.....I couldn' t believe the little rubber tube caused that much verticle drop..............