I'm no expert ... have only been BFing for a few months myself ... but I've had similar issues on and off with my son. A few things I've worked out which relate to us:
(a) wet nappy - If he's got a wet nappy he'll squirm and wriggle. I always change his nap before a feed.
(b) strong flow of milk - near the start of a feed, DS sometimes needs a little 'breather' if my milk is flowing strongly. He squirms, screws up his face, pulls off and then immediately wants to reattach. When this happens I always squirt milk all over him as he pulls off .
(c) milk flow slows - happens more near the end of a feed, but if my milk flow slows down and he needs to suck a little harder, he will sometimes squirm and wriggle.

Most of our feeds took up to 40-50 mins until approx 8-10 weeks, when suddenly 20 mins was all he needed to fill up. Can only suggest you sit down in front of the telly or have a good book on hand to pass the time.

I reckon most bubs probably squirm and wriggle around a bit on the boob from time-to-time, but if he's happy and producing enough wet/pooey nappies, then you and he are certainly doing something right! Good luck.

Andie