The lens/laser/sensor assembly is mounted on a veritcal slide and a focusing
servo runs it up and down until it gets the signal from the disc. It sounds
like the slide mechanism is sticking, if of that design. If totally
suspended by springs, and no vertical slide mechanism, there may be dirt, or
maybe the air blast bent the springs a bit. Anyway, it definitely sounds
like the focus mechanism is sticking, so see if you can find out why that is
happening.
Jon