There could be a hold request on it for another customer. This will prevent a renewal.
You may have used up your renewals. You can renew an item two times before you must return it.
Some copies of our newest titles have copies that are not renewable. These “browser copies” do not fill hold requests and therefore can be found on the shelf when not checked out. If you’ve checked out one of these "browser copies” you will not be able to renew it.
If you need more information, please call your local branch and ask for circulation.