When is the last time any of these cars have been waxed? If it has been a while, that could be the problem. I have noticed on new customer's cars that do not bead, you can get a slightly filmy look, especially on cool and humid days. The directions tell you that cars that are oxidized need to be polished and waxed to get the best results and in my experience, cars that may not be oxidized but have no wax also can leave some residue when using QEW. Usually, since I am doing a detail anyway, it isn't a problem and I always QD even the cars I just wash. I do recommend wax to the cars that need it, so the next time I wash them I don't have that problem.
Protect All is pretty upfront that paint in poor condition needs to be brought back to get the best results from QEW. Again, when wax is nearing the end of it's protection or completely gone, you can get some minor surface residue that needs to be QDed.
You may also be having problems because of low temperatures and high humidity. Also, are you using a two towel method? If you use one towel (folded into 4ths, refolded as needed) to remove the majority of the water (but leaving a little bit behind) and then following with a drying towel, you shouldn't get any film. I get the best results using a terry cloth towel for the first pass and a microfiber towel for the second pass.
One more option: Are you using a fabric softener for your detailing towels? They will leave a residue on the towels that will transfer to your paint when they get wet.