I closed a skylight and floated the ceiling level. A few months later it started cracking and falling, why? I don't think it was too thick.
Mud is pretty forgiving as regarding 'stickiness' but it won't cope with much movement, usually expansion and contraction will cause cracks.
- Could be the materials you used have dried out (framing shrinking etc.).
- Could be temperature related; i.e. expansion/contraction. Did you insulate the space over the drywall?