Ctrl+Shift not working in KDE

[Mark Ovens]

KDE 3.3.2

Any key combination that includes Ctrl+Shift doesn't work, e.g.
Ctrl+Shift+C to Mark All Read in Thunderbird or Ctrl+Shift+D to
Uncomment a line in KWrite.

It is the combination that is ignored; Ctrl+<key> and Shift+<key> work
as expected.

I've looked at all the keyboard stuff in Regional & Accessibility but
can't find anything.

Anyone got any ideas?

TIA

Regards,

Mark