John
I assume it's not removing the copper in places. ??
I only use MEGA stuff, odd bods I know who do pcb's warned me about 'pick 'n mix' sometimes not working properly, but if you've used it all before that should not be an issue.
Could be the board, I believe it does have a shelf life.
I get mine from CPC, I think it is MEGA sourced.
Only time I got other than MEGA was some board off an ebay seller .... cheap ... and not without good reason it turned out...
All the resist had the 'spray' lines across it ... utter rubbish. Fortunately I only got 2 A5-ish sheets to see what it was like.
I reckon I could put the resist on better myself with a stick ...
Dave BC