[dust2dust]

not that strange at all
1 laser lens is used for both but many other parts that produces diff beams to either dvd or cd
it happens all the time

try cleaning with a cleaning disk with brushes on it normally does the trick although dvd are normaly first to fail

try adjusting the lasers cd pot ,there seperate pot for dvd(get a pro to do this

if still no good then it looks like the laser needs replacing