Whenever you reassemble a drum brake on a moped or cycle, don't forget to centralize it first.
After the wheel is reinstalled, tighten the axle nuts lightly, to take up play, but not fully tight. You just want them to have a light grip on the assembly.
Then, spin the wheel fast, and grab the brake good and hard. Do this several times, and then while holding the brake firmly, tighten the axle nuts. This will help to centralize the brake shoes in the drum, and give you better brake action.