Nike Returns

I sent back my full Nike order of 6 pairs of trainers in the two original boxes that they came in. I used the Royal Mail tracked 48 returns label that Nike supplied. Posted them at the local post office and got the two tracking receipts. However I only received a refund for one pair of trainers. I contacted nike after 48 hours to ask where the remainder of my refund was and was asked to supply the tracking receipts which I’m emailed to Nike. I then received a refund for another 4 pairs leaving me short of a refund for 1 pair. After questioning this Nike told me that if the warehouse operative says they didn’t receive the full order as a return they will not issue a full refund. How can i prove that i sent the full order back? Also as the two boxes had more than one pair of trainers in them (2pairs in one box and 4 in the other box) i cannot understand how the can claim not to have received all 6 pairs of trainers??