I'll try to ask the question as general as possible and I will give some spoilers with specific examples below. I've only watched the TV series of the Flash, but I will happily accept answers from the comics.
So the question in general: Is there any story reason why the Flash suddenly is so slow in key moments of the episodes? The non-canon answer is pretty easy: Because it would be boring, but maybe there is some kind of story-answer, too!?
Barry and Joe are driving in the car as Mark strikes the car with lightning. Barry is fast enough to see the lightning move down and is able to get them out of the car. But he couldn't get up and just knock Mark down.
This is the second episode with the "Captain Cold" who fires the cold gun at Barry while "Heat Wave" fires the heat gun at him. Instead of just taking the guns he lets both of them fire at him so the guns would disable each other. He already undressed a robber and put a policeman beside him, but he can't just simply take the gun?
Thanks for your time.