Saman Ghoddos' early header was the difference between the sides, with the Hatters dominating the second half.
Frank said he could not pinpoint the precise reason for the Bees' sluggish second half but believes the Covid-enforced break and closure of the training ground may have played a part.
"We started brightly, quickly and pressed them and had chances to be further ahead by half-time, but in the second half we weren't aggressive enough," the Dane said.
"The second 45 minutes were extremely painful to be part of. In terms of a quality performance for the eye it was a three out of 10, but for work ethic, attitude and togetherness it was a 10.
"We didn't train for a week, so maybe after playing regularly and getting into a rhythm this was why we looked flat.
"But we put bodies on the line, did everything to get blocks in and got the points that we might not have last season."
Frank was baffled by a second yellow card shown to top scorer Ivan Toney, who now misses Brentford's fourth-round FA Cup clash with Leicester on Sunday.
He said: "I don't understand why he got a yellow. I've seen it back and can't see any reason for him to get a yellow."
Luton boss Nathan Jones was lifted by his side's never-say-die spirit as they laid siege to Brentford's goal in a one-sided second half.
"We definitely deserved something from the game and there won't be many teams that come to Brentford and dominate the game like we did in the second half," Jones said.
"In the first half we were too passive and allowed them to play and I was disappointed with that. In the second half we dominated, were on the front foot, pressed, moved the ball well and got into great areas and had chances to win the game.
"I couldn't ask for more. The desire and quality we showed was good and we had enough opportunities and territory to have got something from the game. We wanted to be aggressive and, barring a bit of quality, we would have got something out of the game.
"The fact that we did what we did to Brentford, who beat us 7-0 here last season, proves we are evolving as a club."
The Hatters boss was angry at Tom Lockyer's stoppage-time red card for an off-the-ball clash with Toney.
"It's not a red card or something he should be sent off for. There was no contact," said Jones.
"It was a really poor decision but I'm disappointed my player got involved with a minute to go after an earlier incident. We will appeal it and hopefully get off because he will miss three game and he shouldn't."