Plextor px-708uf

I have a plextor 708 and have burned approx. 110 dvd’s on it with
a few of them not so perfect. Now it shows (using dvd 1-click) that
it is reading, and writing but produces a blank disc. I am very new
to this so I get stumped easily. I have tried different media, and
the lastest downloads from Plestor. Any thoughts would be appreciated.

What kind of media do you use?