Deadball

[dh898]

Newbie here. I was an umpire at a JV game Saturday. We had movement on the offensive line which was flagged but not blown dead by either linesman or the referee. I didn't see movement. The play goes on and A gains a first down. The WH consults with the HL and LJ who both threw the flag, explained the play should have been blown dead before the snap, waved off the flag and let the play stand, saying it was a dead ball foul and should have been blown dead. Since it wasn't, play stands. Was this the correct ruling?
We got roundly booed and the coach went ballistic.