It is highly likely it's stuck in Customs - in this case, just be patient & Customs will clear it. Reasons for slow clearance include incorrect documentation, ripped papers or wrong description.
It's possible the address on the parcel is incorrectly written - in this case, just be patient and Royal Mail will deliver it, or return to sender.
Give it at least 6 weeks from date of posting, then put a claim in to the person you bought it from.
