After gaining some recent momentum, Spurs were thumped 3-0 on Sunday.
Callum Hudson-Odoi struck twice, with Ibrahim Sangare also scoring at the City Ground.
Frank said afterwards: "That was very disappointing. It was a bad performance. Frustrating, because I think the last three games were good on performance and results, normally linked. And you can lose a game like this. That's the frustrating part, of course.
"I think every game, I think, until the 1-0 goal, it was a little bit back and forth for both teams. Hard transition both ways. And then after the first goal, I think we lost structure and were more disjointed.
"And then the 2-0 goal killed the game off. Basically not in the game after that, in my opinion. And that's part of keeping the structure, keep the game plan, keep going, keep doing the right things.
"So that's definitely not the first time we've seen that, this year, last year, whenever. That's something that's ongoing work in progress before we're more consistent in that.
"Doesn't help that we made two mistakes with the first two goals. But again, that can happen in football. So we just need to keep doing the right things.
"So everything looks the right way. And then on top of that, we decide to throw the ball away at least 25 times. So we can't hit each other 25 times. That's going to make it difficult for you to create anything or to win a game."