So you're using the latest nightly, not
my unofficial release? In either case, important to know so I know where to look for the problem. In the unofficial release I did up the focus on gems a bit, so if you're using the nightlies you might already see an improvement by updating to 0.4.6u.
That said, I've loaded your save and can't find any issue. Yes, imps aren't mining because of the locked door, by either unlocking the door or dropping imps inside the area with the gems they will start mining. I frameskipped to see if they would keep doing so, and they did. They never stopped mining.
So unfortunately, this save-game is no help, but if you come across your issue again, please share.
A small hint for specifically this level: You now have your treasure room to the east so your imps have to walk around the rock to get to the treasure room. But you also have room for a walled 3x4 treasure room right next to the gems. (Above the lair where your vampire is now.) That's much quicker so you'll get more cash.