Newcastle won 2-1 via goals from Harvey Barnes and Alexander Isak, with Dan Burn putting through his own net.
Postecoglou said afterwards: "It got away from us unfortunately. We had the chances to win the game probably comfortably but we walk away with nothing so it's a bitter pill to swallow. It is what it is.
"If you don't get the second goal when you have momentum... The keeper has pulled off a couple of good saves but we still had the chances to create something more and we didn't.
"When we're creating the amount of chances we have, we should be getting more in return than what we have been at the moment.
"The positives are that our football is good and has been in the past three games. We just need to get our rewards for our football. If we continue to play like that, I know we will get our rewards."
Postecoglou also said: "Story of our season so far really. We played well and controlled the game for the most part. They were a threat in transition but we still had enough chances to win the game comfortably.
"I thought we were handling the transitions. The goal we conceded in the second half was sloppy and we lost focus. The game should have been put to bed a lot longer before that.
"What I know is if we continue to play like that we will get our rewards. It's not an easy place to come. For the most part, I thought we handled that really well. The way we controlled the game was really pleasing.
"Frustrating more than anything else. It's three good performances but not the results that reflect that."