Why isn't the wax come off?

I've been using warm wax with paper strips for months with no probs, but all of a sudden the wax has become extremely difficult to remove- remaining on the skin after I have removed the strip. I then have to use a blotting method to get it off which takes a layer of skin with it- not good!! It has happened on several clients and when using it on myself. I have changed the wax pot thinking it was a bad batch but it continues to happen. Any ideas?