Bought some new wellies and they feel like they're the right size (a bit tricky to get off by hand but feels like my toes have enough room) but they seem to rub my heels after about half an hour of wear. I've worn them about 5/6 times and always the same - fine for the first half an hour but then start rubbing. Should I be wearing thicker socks/2 pairs of socks or will they eventually wear in? Any advice appreciated from anyone that has these wellies!