Philips DVD Recorder 3355 DVD +R -R

Hello everyone, I have this problem that I discovered recently, I can record with any type of DVD (+R and -R) in this machine and also plays fine, but when I want play DVD Discs not recorded by this machine, (e.g. burnt with DVD Writer on my pc), only reads ok the DVD +R and not the DVD -R (in the last case puts “BLANK DISC” on the display). Is this normal?, I should upgrade the firmwire or something like that??. Thanks a lot.

CP