No one can help.

Seller support employees are either unwilling, unable, or specifically trained not to use critical thinking or reasoning skills to solve problems. I suspect it's the latter.

All you can do is wait. They will eventually receive and check-in the items. If you sent 20 items and they say they received 18 items, they're not going to go looking for the other 2. Most of the time, if there's a discrepancy in count the "missing" units will pop into your inventory at some point. They're usually in the warehouse somewhere, but someone has to stumble upon them and go, "these haven't been entered yet." Could be 9 months later, but most of the time they appear in your inventory eventually.

I had a similar situation. Sent 21 units, Amazon checked in and received and said 0 units had been received. essentially they said they got an empty box. I went round and round with seller support (phone, email, chat) and no one could do anything about it. So I just gave up and figured that was an expensive instance of "the cost of doing business."

I sent the items in January 2024 and in June 2024 I suddenly had 21 stranded units without active offers. Apparently, they found them! Most of the time that's how it goes, they appear eventually.

Be sure to always use the FNSKU labeling process, don't co-mingle inventory by using the manufacturer upc barcode. If your individual FNSKU sticker is on the item, if the item eventually appears, they know exactly whose inventory to add it to. Or if an item is damaged in the warehouse, they know who to reimburse. If you use the manufacturer upc barcode, there's no way to tell whose stuff is whose. Those items are far more likely to be lost or damaged without compensation for you, because there's no way to tell which ones are yours.