brew won't carbonate

My last batch was a Rye beer and the fermentation went as expected.  It is a little sweet but not overly so.  My problem is that it's been in the keg for over a week and it won't carbonate.  I started at about 10psi and after a couple days went to 15psi and now I'm up to 20psi and it's still flat.  I did a quick sanitize on my keg, didn't take it apart, and the last batch carbonated nicely so I don't think it's a mechanical problem.  It pours well, though, maybe not as briskly as it should at 15psi.  My CO2 bottle shows pressure at the edge of the red zone so I have good pressure there.
I'm at a loss as to what to do.  Any ideas would be appreciated.

Try shaking the heck out of it with the CO2 attached at 15-20psi, being careful not to let beer into the gas tubing. Or charge it up to 20psi, detach, shake, and reattach. You should hear a hiss as gas enters the keg. Repeat until carbonated.

Thanks, that worked.