The most common problems associated with fill zones (copper pours) were summarized by @Rene_Poschl just yesterday, in the thread “GND Zones not filling” at GND Zones not filling - #3 by Rene_Poschl :
I posted a footprint for DPAK (that DOES work) in the thread “Pad Holes Under SMT for Heat Sinking and other questions” at Pad Holes Under SMT for Heat Sinking and other questions - #4 by dchisholm . Compare its construction to yours - especially the properties of the pads.
If you still have a problem after investigating these potential faults, post your footprint file.
Dale