He felt his players showed effort and determination but lacked the cutting edge needed to get the points.
Frank admitted the 2-0 defeat was disappointing, as he believed Brentford deserved more from the game.
“It feels like we missed an opportunity to get three points,” said the Bees boss after the match.
“In many ways, it was a good performance, without it being a through-the-roof performance - we created so many opportunities and good situations without having enough cutting edge to create five or six 100 per cent chances.
“We created more than enough to score at least one goal, and then it would have been a different game - that’s frustrating because it’s such small things that decide a match.
“For example, their first goal… we hadn’t conceded from a corner all season. It happens, it is what it is, we had plenty of time to get back into the game but we couldn’t do that.”