“We were not accurate enough in the first half and we gave a very good side a 17-point head start,” Dowson told BBC Radio 5 Live.
“They shut us down at the set-piece efficiently and we couldn’t get the ball away from the breakdown quickly so our attack was stilted.
“This is the first European semi we have been in for some time against a very good Leinster defence and we made some uncharacteristic errors.
“Those things are going to happen but there were a lot of them, but we stuck at it and gave ourselves a chance.”
Premiership leaders Northampton have been praised for their attack this season, but Dowson says he was “frustrated” his side were unable to impose themselves.
He added: “Our defence, our collisions and our effort were outstanding and that allowed us to still be in the game.
“It speaks volumes of the group and it’s clear how invested the players are in each other.
“In the second half, we slowly reeled them back in but we didn’t have enough.”