If a job site block has another block on top of it, villagers will claim it but will not be able to pathfind to it and therefore will not use it to restock.

How to Replicate.

1.  place a job site block

2. place a block on top of the job site.

3. spawn a villager nearby.

As demonstrated below, the villager will claim the job site but will not pathfind to it or restock until the block above is broken.