Milking the cow

Not only is Bell unusually flat-chested for a cowgirl, she also can't produce milk! And indeed, any attempt to milk her would be in vain.

Succubi magic
If Rinny was previous rescued and Lust Doll learned about succubi body workshop, then, after failing to milk Bell, they may suggest Bell to try it and enlarge her chest there.

But not even the body workshop would work: the moment chest grows up, some unknown force immediately shrinks it back! Rinny, sad because of such failure, presumes that someone must have cursed Bell at some point, and promises to do some research on that matter. She also teaches Lust Doll the spell-growing spell, just in case.

Darker magic
After trying (and failing) to solve Bell's problem with Rinny's help, you may suggest to visit Pixie instead (requires finishing Manor quest first), as she's a powerful mage.

However, despite all efforts, Pixie wouldn't be able to achieve results either, only confirming that it's indeed a curse. If not even Pixie can do it, then who can?..

Currently, this is where this quest ends.