according to videohelp.com, under the DVD Players section, the PS2 *should* play -R and +R discs fine.

what burning software did you use to burn the DVD?

did you include both an AUDIO_TS and VIDEO_TS folder on the disc? (i know for some players, they 'need' to see an AUDIO_TS folder in order to play the discs without problems.)

have you tried using a different brand of DVDs?

do you know what version your PS2 is? apparently there are something like 6 or 7 versions of the PS2, so this might come into play in this situation.