Matte printing poor, but plain paper OK? Canon S820

I don't know why, but my S820 has started printing matte paper with poor
results hardly any blue in most cases, while printing out perfectly in plain
paper mode. The setting for "plain paper" works perfectly when using matte
paper (although occasionally, it will print in true "plain paper" mode on
the matte paper, rendering a worse than matte paper image).

All of the ink tanks are OK and I suspected a driver problem, so I
reinstalled drivers from the Canon website. I'm on a Mac G4 running OS 9.2.2
(I also have 10.3.4 but haven't experimented with printing in OS X yet).

While I suppose that I can get by and simply print on matte paper using the
plain paper setting, I wonder if anyone can suggest what might have happened
to cause this problem with the printer. It seems as though somehow the
printer "forgot" how to print to matte paper, while nevertheless doing so
appropriately in the "plain" paper mode (at least *most* of the time!)

Thanks for any help!
Paul