The Bucks fell to a 4-1 defeat at Gloucester City on Tuesday, having put in an impressive first half showing before Brendon Daniels was forced off just ahead of half time.
The Tigers went on to claim victory with a punishing second half display.
Cowan felt that his side struggled to maintain their performance, and admits the players must put in more complete showings.
"We came with a bit of a gameplan and Dom McHale was getting lots of joy and then obviously we had the injury to Brendon which stunted our growth somewhat." Cowan said.
"It was really disappointing. We went in at half time feeling very in control of the game and very much in a position where we thought we could go on and win it.
"We're having a problem at the moment of being consistently excellent, but you can't put in performances for half the game especially against a team like Gloucester and unfortunately we couldn't really bring our A-game in the second half and we had to change shape and change personnel and the game plan went out of the window somewhat.
"But there's no excuses, it's still 11v11, we were still matching them up. It was just that we were so in control, passing through them up until that point.
"There's a reason why they're up there and they do things consistently excellently, whether it's a five yard pass or putting it in the right areas or making right decisions. We did that brilliantly, better than them first half but second half we had a bit of a mad 10 minutes.
"We feel a little bit aggrieved in certain areas, but we have to take responsibility for the fact we've got to put a whole performance together.
"It's disappointing but we're going to have to find something from somewhere."