The first thing to do is to check if the parcel is actually on it's way back to you. It may even be out for delivery. You can do that here:
If the parcel does not seem to be moving, or when issues arise with parcels the quickest way to get a response is to first complete form the below:
Once this has been submitted CollectPlus will investigate the cause of delay and ensure that the parcel continues its journey where possible. If the parcel is deemed lost then they will request that a claim is completed.
If 10 days has passed without delivery please make a claim