Its a different story when there is weight on the bike than when its on the stand. There is a reason that drum brakes arent on the majority of bikes. They just dont work that great. I would suggest tightening the adjustment just till they start to rub. To where it will slow it down while on the stand with no brake applied. Ride around and try that.
In addition, if you got any lubricant, grease or chemicals in the brake hub while you were working on it the pads may be slick or softened.
Then there is always the chance that when assembling it you didnt have everything seated correctly inside the brake hub and when you went to ride it, the pads or actuator may have fallen out of place. This is possible but not likely.
Hope this helps.