GB , are these existing customers that your getting probs with or new accounts ?
If its all the above then then i reckon its down to the fact that you in the past have been a glass only man , i could be wrong , but so many times in the past ive seen this same thread come up .
When trad i saw fit to name my company panes & frames , this was 6 year ago , i therefore on each visit did as i promised and did not cut corners , this is why i was full up in 2 months of hard graft , more importantly i reckon when i went wfp 18 months later i had absolutely no probs with spots or runs .
The only advice i can offer to you is to bite the bullett and spend lots of time on every job scrubbing and rinsing.
You will earn naff all for your time to start , but thats the joy of change over , just glad i had my old man drumming the line "if a jobs worth doing , its worth doing well " all them years ago , cheers dad .
Rich