I had a similar problem with my Casio Solar Triple Sensor watch; battery indication showed it was charged, but when I tried using any of the higher current functions like backlight or magnetic bearing compass, it would wink out and reset. I charged it for days under my CFL lamp, so I figured it was charged, and the battery must be bad.
Tried contacting Casio, but they never replied, and I couldn't get them on the phone. I was about ready to write the Casio off, and revised my opinion of Casio customer service, which I formerly thought was good.
What I finally did was give it a *really* goood charge under full sun, and that brought it back to full operation.
The charging process really needs a good long charge under direct light if you let it discharge way down; it's definitely not a quick thing. Forget about charging with a flashlight, that just doesn't do a thing. If you look at the specs on how long it takes to charge, even normally with direct sun, it's a lot longer than I expected.
Anyway, leave it outside in the sunlight if you must, not just through a window, which filters out the high energy photons you want to capture- it should come back.