They might be oxidized like plastic car headlights get after they're in the sun for a few years. When the lenses get oxidized they only get yellowed on the surface, and there are kits at most auto parts stores to remove it. I never tried the kits myself so I can't say for sure that they work, but it might be worth a try. I think the kits come with some buffing wheels for a drill and some kind of polish and wax.
DONT USE PAINT THINNER! it makes clear plastic turn cloudy white! Those kits I just told you about aren't expensive, i think they were 10 or 15 bucks. I think any solvent that will be strong enough to take the oxidiation off will also damage the lense, so i wouldn't try anything unless it's made for clear plastic.