It is possible that the shoes are not in full contact with the drum due to the turning. I would check the shoes to see it they are showing a small contact patch.
It used to be standard practice to 'arc grind' shoes to match the diameter of newly turned drums. I haven't seen this done in a long time. If this seems to be the problem, you have some choices:
Live with it until the shoes finally wear in, just normal wear will eventually fit them into the drum.
OR
Search out a shop with the correct machine to do the fitting.
