Bill,
Aside from the fact that you can only "Paste" if the Windows clipboard has something in it - that means you've already done a "Cut" or "Copy" to fill the clipboard, the "Paste" option only appears if Internet Explorer believes that the clipboard is ready.
Copying text from one location to another, maybe changing between applications too, has been known to empty the clipboard. Trying again after both applications are running usually fixes this issue.
The Cut and Paste items are also "type" dependent - if you have a graphic image included in the "Cut" then a text only field - like the PC Post Message dialogue - will not show the Paste option. And yes I know images can be displayed on this forum, but Post Message is text only (and images are referenced by their name and location)
And finally you can try holding down the [Shift] key and tapping the [Insert] key - this keyboard shortcut often works better.
Robert