Villagers aren't restocking their trades

Put the summary of the bug you're having here

What I expected to happen was...:
Villagers should restock up to twice per day when provided with an approprriate workstation and a way to get to it. There was magma under the workstations but I tried removing it and it didn't work. There is a zombie nearby (for curing purposes) but the villagers restocked once or twice and then stopped so I don't know if that's the case. 

What actually happened was...:
They don't restock at all, I waited for days. I'm lost, I tried everything

Steps to Reproduce:
1. Put Villagers in a 1x1 cell with the workstation right in front of them
2. Buy out an item so it locks
3. Wait for them to restock and repeat a couple of times
4. They don't restock, at all