Some drives just can't read some formats. Very old drives(say older/cheaper audio CD players) can't play CD-RW's(or some can't even play R's) My old laptop couldn't read DVD+R but DVD-R was fine. So yeah, as some people said the firmware could help out...
DVD+R is suppose to have more features on them, I think to make them faster, perhaps even burn better. However, I have yet to find a real advantage over DVD-R's as DVD+R's are less compatable in drives... Many DVD video players won't read DVD+R's