EDIT:
I didn't see the second page of this until after I had posted - seems like you have your problem solved. The first board I see a picture of seems to be slightly over exposed, and the board wasn't in contact with the artwork properly - one of your traces has thinned a little, a sure sign of it.
That white PCB board is the same as I use, Economy Photoboard II
Good choice
My original post:
Sounds like the board you're using is a bad batch or way past it's shelf life. I use either "economy" Photoboard II" (from Mega) or FR4 from Rapid electronics. The resist surface is green, and goes a sort of purple-green colour once exposed. I expose usually for around 7 minutes (home made light box, using two blacklight CFL's) and develop my boards in Seno developer - it's non caustic by all accounts. I pretty much get perfect results every time, only time it messes up is either because I've forgotten to mirror my artwork (but the board still develops and etches perfectly) or the edges of the board have uneven coating of photo resist (but this is a manufacturing issue).
Recently I have taken to using the Seno SN100 photo resist applicator - and as long as the copper is clean (and I mean CLEAN) and you get a fairly even coating of the lacquer, all works pretty well. Exposure and development is as Photoboard II.
By all means try the spray on stuff (Positiv20), but know now that to get fine quality high resolution work, the makers recommend you to bake the board. It also stinks like you wouldn't believe and to get an even coating is next to impossible unless you have experience using a spray can. Too thick a coating and it'll run and curtain. Once it's at that stage, you need to strip it all off, clean the board and start again - the Seno applicator is so much more simple to use, you just wipe it on.
Again, I will repeat the golden rule - make sure the copper surface of your board is CLEAN, and by clean, I mean use a scouring pad on it until it the copper gleams. Don't use Brasso or something like it unless you can clean off ALL the residue - including any fingerprints. Photo lacquer simply will not adhere to a mucky board, and the resist WILL flake off in the etch bath. The same is true of the etch resist marker pens that you can get - the copper MUST be clean.
Developer that is too strong or too hot will strip unexposed resist. Under exposure will also cause problems, but it will lead to parts of the board not etching, since not all the resist will be removed during development - it certainly won't lead to tracks dissolving or the resist flaking off in the etch bath. OVER exposure is what leads to tracks being etched, UV leaks through the blackened out areas of the artwork! I suggest that the board you are using is either faulty, or just old stock - photoboard DOES have a shelf life, as does camera film.
Obvious as it seems, don't handle photo board or apply photo lacquer in strong daylight, do not develop it under a fluorescent tube light - it will get exposed, and the developer will remove ALL the resist. Seno SN100 can be applied on a dull overcast (rainy) day - at least I have never experienced issues doing it.
I've been doing it this way for 30 years, honestly, it does work the way people here are telling you.