Richie Barker admitted his Crawley players were below par following their 1-1 draw with lowly Colchester in League One on Tuesday.
Anthony Wordsworth's late penalty denied the Red Devils victory after Kyle McFadzean's controversial goal had put the visitors in charge.
McFadzean appeared to handle the ball before putting Crawley ahead with 19 minutes to go, while Mike Jones twice hit a post for the visitors.
Barker said: "I thought we played within ourselves in the first half. I said to the players at half-time that I thought we played in second gear.
"I don't know why; perhaps it was a bit of tiredness or the fact we had had an excellent away win at Preston in front of a big crowd last weekend. Perhaps it was because Colchester - with all due respect to them - were bottom of the table.
"Whatever it was, they found it difficult to lift themselves. That was disappointing but it was better in the second half. We hit the woodwork and sometimes, it's just not your night."