If you get an e-mail from the library about your item(s), and it specifies that such and such an item cannot be renewed, it might just be BS. Go to the website and try it anyway. In my case, I got a renewal on the item, even though the e-mail told me I couldn't. (And my panic subsided; I'm out of province!)
Here's what I think happened: The book I had out had 14 copies in the system, of which 5 were checked in, and two holds were placed. The e-mail said "The item marked with "***" may not be renewed. Others have requested. this item. [sic] This item will need to be returned on or before the due date." So evidently the e-mail notification program is not sophisticated enough to handle instances where there are multiple copies of one item.
Of course, the e-mail could be right, if: 1) The number of holds exceed the number of available copies. 2) Someone puts a request on that specific copy - this happened to me once with a novel I borrowed for school, and it was really annoying - I guess someone must have really wanted the hardcover! The program would have to be written to check for these conditions as well as the number of extant renewals before saying that a renewal wasn't possible. A lazy programmer would just have it check for a positive integer in the holds field and then have the e-mail unceremoniously state that a renewal is impossible because, truthfully, other people have requested the item. And it will say so even when there are plenty to go around.