Sometimes MATLAB can get confused and you need to rehash. This can happen in various ways, including if a file that is not on the current path is modified, such as if you are cd'ing back and forth between directories.
If you are adding files to directories that are underneath the MATLAB installation (not a good idea) then rehash toolboxcache
In some cases involving network drives, MATLAB might not get notified when a file is updated, and you might need to rehash
If you are modifying a .m file which has been invoked in the current session, then MATLAB will not always notice the change, and you might need to clear the function name.