Usually, an average Windlass is designed for picking up a weight of 3-4 shackles + Anchor weight. Beyond that, it is quite impossible for the windlass to heave.
Looking at the present situation there is a possibility that the anchor is holding the ground through ONLY the flukes.

There are three possibilities to it -

1. The flukes might be stuck on to an obstruction, check chart and local navigation warnings and information to rule out this possibility ; or
2. Try to couple the power of both windlass by rigging the mooring rope from one windlass to another; OR
3. Let go the anchor from the bitter end. Attach the buoy to it and mark the position, inform the authorities and nearby ships.