On STK, what action should I take if items are still "processing" after 3 months?

Fulfilment By Amazon Shipping Inventory to Amazon
1

STK reads:
AVAILABLE: 21
INBOUND: 0
PROCESSING: 1

2

This is an Amazon issue. Download the Reserved Inventory report from Seller Central and then raise a case with Seller Central Support. Alternatively, get in touch with SellerToolKit.