ob commands don't cache

[vaibhavsagar]

Running e.g. ob shell results in a rebuild each time when I would expect it to be cached after the first time, e.g.

vaibhavsagar@nixos-l14g3:~/repos/uptrust$ ob shell
./.obelisk/impl: command not cached, building ...
✔ Built on ./.obelisk/impl [command]
Hint: use '--no-interpret path/to/dependency' to force building an unpacked dependency and include it in this shell.

[nix-shell:~/repos/uptrust]$ exit
vaibhavsagar@nixos-l14g3:~/repos/uptrust$ ob shell
./.obelisk/impl: command not cached, building ...
✔ Built on ./.obelisk/impl [command]
Hint: use '--no-interpret path/to/dependency' to force building an unpacked dependency and include it in this shell.

How do I figure out why this is failing to cache?