Photoshop shortcuts only work sporadically Using CMD+J on a layer to duplicate it works... sometimes. But not always. 

Using CMD+Ctrl+D (my own binding) to delete a layer works... sometimes. But not always.

I'm unable to grasp why it only works sometimes. I noticed switching tools sometimes makes it work but that doesn't always help. 

Anyone experience anything similar?

I'm using Mac OS X Snow Leopard, Photoshop CS5 12.0.2 (English), a few custom shortcut binding but not many.
One of my peeves about InDesign is that the document navigation shortcut keys (e.g. alt+spacebar) are tool/context dependent instead of globally available. Perhaps you are experiencing the same issue here. Also, some Photoshop activity requires you to commit or cancel the change (such as using the transform gadgets). While in this state, certain actions are not possible.