I've found that's pretty much normal with DPD these days. They used to be one of the best, but have been hopeless for the last year or so.
Have you asked the seller to chase this up with DPD? That would be the first ting to do.
If it is now beyond the date eBay told you to expect it, you can open an "Item Not Received" case. This doesn't really help if you need the wallpaper urgently, but it may prompt the seller to chase a little harder, if he has already done so without result.