If it's related to the loading then heavy bath towels could still cause problems. Basically they have out of balance protection systems which monitor the motor to make sure the load is well balanced. If it isn't because of not enough items in the drum or it can't balance heavy items it will refuse to spin or will stick for ages on the spin cycle trying to balance the load. However, if this is the problem it should work ok at least sometimes with nice even full loads.
Check out these articles in case it helps, even if it makes you think loading isn't the problem - Washing machine wont spin just one item or very small loads. The out of balance protection (which is too sensitive on many washing machines) is designed to protect the washing machine from banging around violently on spin. This article was written before the prevalence on new washing machines with this system fitted but contains advice about how to get a well balanced load - How do I avoid out of balance loads?