hi,
we have had a similar problem with some heavy rolls. Are you using the plastic inserts? They help, make sure brake is off, and we always unravel a couple of meters of material to help with the weight issue.(and with some heavy lonas, manually pull when it is running through)
If it is happening all the time: motor error message then speak to the person you bought the machine from, for instructions on ho to check if the motor is knackered it can happen.
the other option is to disable the automatic pull through option, but if you do this you must make sure that A you have enough media, and B it is alligned correctly if it is a long or very wide print.
hope that helps, it drove me barmy for a while!!
Dan