During the course of a swim, my freestyle starts off a bit stiff, then gets smoother, then I might hit a super good patch with some great new feeling in whole body connection or catch etc. Then a few minutes later, it fades away.
The body has its own natural intelligence. I like to think the trick is to pay attention to when you get the brief good feeling, don't overanalyse it, and gently try to work back to getting that feeling again.
When my catch is a bit weak and useless, I can swim further and get less out of breath, so it's nice and relaxing.
When my body works out how to get a firmer catch, it takes more energy, and I get puffed out earlier, as my fitness gradually improves.
So, weak catch or firm catch is no problem, it just means my swim is different at that time.