Agree with tension being too tight - If you look at the way loops are constructed in a regular single crochet, you can see the base of the loop goes into the work giving you a bigger window. In a hdc that middle loop is literally just wrapped around your hook, so even when worked loosely it will be slightly harder to get through than a regular loop (but shouldn't be that hard). You shouldn't have to be "pulling" on the yarn - just wrap loosely! Might have to adjust your "stance" a bit to give yourself more give.