it's because there's nothing to redo.
Maya's undo stack is VERY fine.
So fine that selection is undo-able and redo-able
eg, if you hit undo then click somewhere, anywhere, then it will kill the redo stack because that click has made something happen on the redo stack
so, you're probably doing something like clicking somewhere or moving the camera after the undo. You'll just have to be more aware that every little thing triggers it. it's actually quite useful, especially if you accidentally lose your selection.
that's a "Ch" pronounced as a "K"
Computer skills I should have:
Maya, XSI, Photoshop, AfterEffects, Motion, Illustrator, Flash, Swift3D