A quick reassemble as Julian has indicated will certainly improve matters. Here are a few other things to consider on any cam operated brake.
Relieving the cam end of the trailing shoe lining will allow the leading shoe to have a bit more pressure as the cam can continue to turn, as T T says. Mugwump suggests 1/3 of the length, so start off gently and see how it goes.
The drum operating lever should be set so that it never passes over the 90 degree angle with the cable. Make sure there is no stretch in the outer cable, as this is is wasted effort. Worth checking you also have the correct handlebar lever, as regards mechanical advantage. Make sure the floating pivot is set up to work as intended, either fully floating or locked in place with the brake applied and the shoes centred, depending on type.
Once full contact is achieved, by bedding in, sanding off high spots etc, the poor performance can only be the retarding properties of the friction lining material.
Swarfy.