If they've lost an entire shipment, chances are that it has simply been misplaced and you'll get the inventory when they eventually find it. They lost an entire pallet of inventory a few years back and it took around 3 months, but it eventually showed up in my inventory.
If they're only asking you to prove up units you've sold over the last 12 months, they're tightening that requirement. It used to be that you'd have to prove ownership of enough inventory to support every unit that you had sold of an item from the beginning of time until the date of the shipment...and if you're even 1 unit short, they'll deny the entire claim. The requirement is a total PIA, but makes sense if you think about it. If I've bought and sold 1000 units of a given item and decide to gin up a reimbursement claim by including another 100 units on a shipment that never show up to a FC, this requirement prevents me from using the invoices for the 1000 legit units to support reimbursement for the non-legit 100 that I'm trying to scam them on.
One upshot of this type of situation can be that you can get reimbursed for the inventory AND get to sell it if it shows up well after that reimbursement has processed. I'm not sure if they've put things in place to prevent this from happening now, but I've had a few instances over the years where I got to double-dip. On one occasion, I even wrote to them to tell them they'd already reimbursed me for the inventory and they told me to go ahead and sell it.