Brand New MacBook ejecting blank DVD-R and DVD+DL

Hi,

I have a new MacBook (about 2 months) and am trying to burn DVDs but it keeps ejecting the blank DVD (about 5 seconds after I put it in). I have only burned 2 CDs in the past, and without a problem.

I tried both DVD-R SL and DVD+R DL from Magnavox and Memorex, respectively.

Any idea what is going on, or what to do?

Thanks!