Are Certain Magical Item Not Disenchantable Skyrim?
You may neither enchant nor disenchant staves.
Additionally, you may not disenchant unless the item has an enchamtment you don't already know.
Additionally, there are some quest rewards you may not disenchant (for example: Ghost Sword).
And finally, there are some items (such as the Amulet of [Insert Aedra here]) that you simply can't disenchant.