Greetings,
     I asked the very same question about one month ago.  We are experiencing 
this artifact in a variety of ways:  ie. special stains (Congo Red, Giemsa, 
Fx Muscle stains, Iron stains....decals turn almost a chalky white, and some 
ipox.)  There are times I can actually see "it" develop and spread before 
having to remove and redo the coverslip.  Indeed, every now and then I have 
to recoverslip some slides 2-3 times!  I received advice/suggestions ranging 
from not enough xylene on the slides as they run through the coverslipper, 
trapped air, to loading only one rack of slides at a time with xylene 
dripping from them.  We are now going to try putting a little dessicant in 
our dehydrating stations.  Interestingly, a salesrep for a cover film said 
that it seems California is where this phenomenum is happening the most.  
Recoverslipping can cause a real time crunch at the most inopportune times!  
And recoverslipping 2-3 times for the same slide makes me scratch my head!  I 
don't recall this kind of problem popping up quite like this and I would love 
some real answers. Good luck.....and I hope to read more responses myself.

Deb King
Sacramento, CA