Liverpool were preparing to introduce Ryan Gravenberch, Kostas Tsimikas and Rio Ngumoha when the ball went out of play, but the changes were repeatedly delayed.
Fourth official Tim Robinson appeared to be having problems with the electronic substitution board, with Liverpool waiting for around three minutes despite the ball going out on three occasions.
The third stoppage proved costly. Forest took a quick throw through Liam Delap, allowing Neco Williams to run behind the Liverpool defence. Alisson brought down the wing-back as he attempted to get a shot away, and referee Samuel Barrott awarded a penalty.
Morgan Gibbs-White converted from the spot in the 70th minute.
Iraola questioned why the substitutions had not been allowed to take place before the quick restart.
"Definitely he was having some problems with the board or something," Iraola said. "It definitely would have stopped because I wanted to sub, but it didn't happen.
"There was a quick throw-in because we were waiting for the sub."
He added: "A professional fourth official cannot take three minutes. We'd already given him the subs two or three minutes before."
Liverpool eventually rescued a point when Víctor Munoz equalised in the 82nd minute, but Iraola was disappointed that his side had again failed to turn a comeback into victory.
The result leaves Liverpool with two points from their opening two league matches.
"The first half has been poor from our side. We haven't been good enough, we haven't controlled the game," Iraola said.
"We improved a lot in the second half. We were much more intense, much quicker on the ball, had more sense of urgency, much better, but it hasn't been good enough to win the game."
