It's not at all uncommon for parts which are vendor-supplied to occasionally be on back order.
I encounter that from time to time. I have some on back order now, as a matter of fact.
Magazines are especially selling well at the moment, it seems.
My last couple of revolver parts orders were filled pretty quickly, but assorted parts for various 3rd Gen & M&P Series pistols have been on back order.
When I'm able to think of it I try to keep extra parts on hand for repair or preventive maintenance purpose without waiting to find out that I've 'used up' my existing spare parts. Sometimes I'm successful, and sometimes not.