signs of complete PCB photo-resist development
The problem of layouts printed on transparency is the poor density of the black areas. Some UV light is getting through the black color and the resist is exposed partially and thus not fully removed during developing. Try to get better print density by multiple printing or printing with black, magenta, cyan and yellow together. If you cover a unused part of the PCB with a really dense black material you will see if this part behaves better during development and etching.