Has this rug been an issue on any other cleaners? It could be that it jams the brushroll, causing the machine to cut off for safety.
Is there any bleed valve to open on one of these, on the head I presume?
If its just that one rug, id say thats your cause. Can the brushroll be switched off on the cordless models? If so, does it still cut the machine out with the brushroll off?