Homebrew PCBs

I've got a somewhat new tank of HCl etchant that I've used a few
times, but it's mostly been sitting for a while.  It's only pale
green, but it's emerald not brownish.  My test boards (one just
scrubbed copper as a control) have been in it for 40 minutes now.
It's warm and the bubbler is running, I even added a shot of peroxide
ten minutes ago.  It etched real quick the first time I used it, when
it was fresh.

What's the most likely cause of this sluggishness?  What could I try
doing?  Things to test?

I was thinking of making some copper oxide (the wire+heat trick) and
try to get the solution darker, but I'd rather not put too much copper
in if there are simpler ways.