I bought a round (Matlock area) and this is the 4th clean I've done (monthly). They were all trad and I do them wfp. I don't clean in the rain as I don't want to upset anyone but today (foggy damp) I started out and 1 said no and another said "You're not doing them today are you?". So now I'm pee'd off and back home 
Point no 1: When you changed them over to WFP, did you give them a leaflet explaining the system? If so did it include a rain guarantee?
Point 2: In answer to the 'not today, thankyou' (especially when it's only foggy) Tell them you need to keep working or you'll have to give it up because you can't earn a living sitting at home. Then tell them the system works just as well in any weather conditions, but if they don't believe you then they can look when the windows have dried, and if they can see anything wrong with them they can phone you and you'll come straight back and do them again.
If they still refuse after that, tell them you won't be doing them anymore because a customer who won't have it done is worse than no customer at all.