S.L.
Someone else had this same issue a week or so ago & here was my response to her:
I had this problem too & it drove me nuts! I talked to my father-in-law because he works w/swimming pools, waste water treatment plants & just about anything having to do w/water. He told me that I may be using inappropriate amounts of detergent ect & that maybe I should try the cascade powerpacks (I'm sure that you have seen them - they are premeasured detergent/etc pillow packs). So I tried those & have been thrilled w/my clean dishes ever since. They may be a little more expensive than the box of detergent, but it helps me to keep my sanity when I can pull a spot free glass out of my dishwasher!
Hope this helps - otherwise - good luck - I know how you feel.
~S.