I find that a lot of the time if I copy some text for example and then close the window being copied from, the paste function does not paste anything.
I've noticed this in Ubuntu 9.04 right up until present version. Is there a work around for this? Is there any reason why the developers are not looking into this?
You may want to review the Ubuntu Wiki's article on clipboard persistence, as it discusses the problem and how to get around it using clipboard managers.
This issue also happens to be a well-known, 7-year-old bug on Launchpad. If you have a Launchpad account and feel like it, you can go there and mark yourself as affected, though I wouldn't expect it to get fixed anytime soon. This comment on the bug explains why it exists from a technical standpoint.