Today I found a strange behaviour,
My custom list and library ribbon buttons were working find within the list page, but as soon as I added a list view web part to the home page or any other web part page - the custom buttons were gone!
Only the OOB buttons were available.
After some fishing aroun the code I learned that the list view property "Show Toolbar" had something to do with that. Once I changed it to "Full toolbar" my custom buttons were available!
Strange though, since the other OOB ribbon buttons appear all the time... well, what can you do?