super drive in iMac not working

My iMac [late 2009] is not accepting blank CD's for recording. Just spits them out again. Have used [recorded] a rejected disc successfully on my laptop . How can I find what the problem is? Went to disk utility but only asks me to insert a disc for erasing.

One of two possibilities: Either the lens is dirty or the drive is defective. Buy a can of compressed air at any store that sells computer accessories and blow a couple of short blasts in the open slot of the Superdrive. If you're lucky, that may do it. If not, you'll probably need to change the drive out. Slot loading drives have a short life compared to tray loaders, but that's the way it is when you own a Mac.

Thanks will try that