You are bang on with that. I have a typhoon and yes it does have a small knot in the stitching and this is what I too believe is the problem. Gonna go with either replacement latex seals or dry gloves and see how I get on.
How old is it? You might be able to get typhoon to replace it free of charge if it's less than a couple of years. If not it shouldn't be more than £30 and more like £20 if whoever is doing it is reasonable.