Why is it that some machines will be able to read DVD-R and not DVD+R?
And if a drive starts failing to read media that is was able to previously is this the drive or firmware?
'Machines' are not drives. This is because older ones do not support all formats.
The pickup/drive, not the firmware!
The compatibility bits at the lead-in section of a disc do not contain same value as a DVD-ROM.
Signs of a defective read head. Consider replacement.