Usually, when you have trouble working the next row, it means that you either worked the previous row in a twisted manner, or are trying to work the present row twisted. Make sure that you're inserting your needle into the front leg of the loop--from right to left for purling and from left to right for knitting. You can check out the videos to be sure.
Also, as was pointed out, don't tug or snug up your stitches. Just keep them relaxed.