im not much of a troubleshooter but will try:
* r the inserts soaked when u pull them out to wash?
* have u done the waist too tight so itd leak?
* have u been washing with only small amount of washing powder?

As u dont seem to have as much trouble with fitteds, maybe just use them?? Ive learnt that 2nd hand nappies sometimes r better than new

Sorry im not much help. Hope u find a solution soon!!