The question is analogous to "there's something wrong with my car, what do you think it is?".
It's impossible to even guess at the cause without more detail.
What sort of jam / malfunction? Failure to feed, 3-point jam, failure to extract, failure to eject, double-feed, etc., each with subvariants.
Is it always the same kind of malfunction?
Is it only with one specific magazine or with all magazines?
Is it always the same round (eg, last round in the mag, etc.)?
Is it completely random in type and frequency?
There are more questions, but that will get us started. Pix are usually helpful, too.