I'm assuming they are electric drum? Are they "easy lube"?
Could be glazed shoes, grease contamination, broken component...
How long has it been since the drums were off for wheel bearing greasing and brake inspection/adjust?
Anyway, I'd be for pulling the drums off to see what's going on. If it were me, while I had them off I'd go ahead and do repack, new grease seals, and clean and adjust the brakes.
Both Dexter and Al-Ko have very nice downloadable pdf manuals for free.
Under "Service Information" select complete service manual.
http://www.dexteraxle.com/products___literature
And here, just click on "Owner's Manual".
http://www.al-kousa.com/
It is worth it to save both to your hard drive.