I believe your problem is the proportion of water to ferric nitrate
you are using. I use 1 lb of crystals to 2 cups of distilled water.
You can use less of the ferric nitrate and get a slower etch. I use
the stronger solution and watch my etch closely when the solution is
fresh (check after 30 min as it may be done). It seems to me that the
resist--PNP Blue--holds up better with a strong quick etch than with
a slow weak etch. Of course as the solution is used, it continually
degrades and you need a longer and longer etch time, until you need
to make a fresh batch of etchant.
Good luck with your project!
I mix my ferric nitrate 1 to 1 ferric to water.
I also put my silver face up in a pyrex flat pan and put that on a
piece of steel that is on a heating plate.
I find the addition of a little heat helps the process immensely.
However, the heat is also hard on the resist so don't go to hot.