Actually, elyse166, the behavior you are describing is most consistent with not having copied, in the first place.
If you have nothing copied (nothing in the paste buffer), you will see no option to paste, since there's nothing there.
I know. You are sure you copied the text.
Are you using the popup menu, or a keyboard shortcut, to copy?
How about trying to Cut the text (then paste right back, to be safe).
If the Cut operation clears the text, but you still cannot Paste, perform an Undo.
This is simply a test to help narrow down the potential issues,