Baffling Faucet Drip

The cold water shower faucet (old style with separate faucets to mixer) continues to drip after replacing both the seat and washer. This is a trivial job I've done many times over the years and I'm baffled as to how water can still be getting though.
I've tried a few different kinds of washers (flat and beveled), checked that the seat is tightened firmly and not cross threaded, swapped it with another new one though it looked perfectly smooth, and coated the seat threads with sealant. The valve stem screws down firmly, i.e. is not stripped and the washer area looks perfectly fine.
I may just have to replace the valve but have to go through a wall to do it, so any ideas would be greatly appreciated.
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Phil A. Buster wrote:

This is extreme, but take the seat out again. Clean the threads and coat with Loctite. (You can use one of the grades that doesn't become "permanent".) It's also worth sealing the bibb screw that holds the washer in.
Jim
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Thanks Jim. The seat threads are coated with a plumbing thread sealant. I didn't think of sealing the bibb screw. I'll give it a try.
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