How should I say if i can not follow or understand someone?

I remember a sentence but i am not quite sure, I can not catch you, ritght? how should I say that?

I think you mean something like, “I didn’t catch that, can you repeat it?”

Hmm, may I use “it” instead “that” ?
“I didn’t catch it” ?

I guess you could, but it sounds a lot less natural to me just saying it by itself. If you were going to say “I didn’t catch it” you’d need to say something like “What was that word you just said? I didn’t quite catch it”.

Or do you mean something like “I can’t catch up with you”, meaning that I have tried to contact you, in person or by phone, but am never able to do so?

“You what, mate?”

I’m sorry. I didn’t catch what you said. Could you repeat it?

This is more the polite sequence they use in the business world, but you can say it to other people as well. If you say “I’m sorry” it softens it a bit.

You can also add in “quite” like Nathan suggested.

Uh, can you run that past me again?

Sorry, what did you say?

Do what?

Say it again.

What was that?

Come again, mate?