I don't remember having this problem for the first 40 or so posters I did but in the last year it keeps cropping up. I tile inkjet transparencies from my HP photosmart letter-size printer. When a couple of them overlap and if I've cut something out with scissors I'll often have a line that doesn't expose completely in the emulsion under the edge of the sheet. So I go batty patching up my screen with tape before I print.
Just add on exposure time maybe? The emuslion is new and it's happened with a couple different bucketloads now...
My "exposure unit" is a 500w halogen utility lamp.
Thanks for any help.