is ur wheel alligned?
is it axel nut properly fitted anbd tightened..
is ur chain properly tightened? and adjusted,, do the marks on either side of the chain adjuster on same oint ?
if they r not ur brake shoe is not making a full surface contact,,its making a partial contact,,
stand ur bike on double stand and rotate the wheel .. check there should be no sound when no brake is applied,,
also use a good brake shoe,,
i also had this issue when i used cheapo brake shoe ,, so now in my 70 i use a 150 rs brake shoe,,
there is one that costs 250 i will try that but this one is awesome,, lasts longer doesnt needs frequent adjustement,,
so there can be many issues,,
btw how much u paid fore all the job?