Frank said: "We know we always have a chance to come back and we came back. We knew there was a way back into the game.
"Unfortunately we gave the third one away and that killed the game a bit. We played a West Ham team who were better than us today and we didn't perform to our highest level.
"The frustration is we performed badly over all. It seems after a very good performance against Manchester City, it's two steps back again. It's an inconsistent season so far.
"These players have a fantastic mentality and work ethic but we didn't have a high enough performance today.
"We lost 4-1 so that (penalty appeal) didn't decide it. The penalty shout at 2-1 was a clear penalty on Reguilon.
"I'm irritated by the inconsistency of performances. I know it's there. I'm convinced we'll find it quick again."
On Everton having four points reinstated from their original penalty, Frank added: "I'm unbelievably irritated with our performance today so that's my only focus."