Jonathan, I take it you did not dicuss access at point of quote with these customers.
Reading between the lines are you just window cleaning at the weekends for the time being?
You say that they were not in as "arranged"....was this an arrangement you had made with them, or do you mean your own schedule?
I presume they are all inthe same area?
Personally i would draft a quick letter to all concerned, giving them a dated timetable of when you will be calling for the next few months, explaining that you do need gates left unlocked for access and should you not be able to gain access, you will clean what is accessable and adjust the price accordingly. (there will be some on here who will say charge in full, but you are still building a round at the moment and with this amount on your doorstep be flexible.)
Some of these custies may phone and say that their date is not good for them, so as you have 2,4,8 week schedules for these you can afford to be flexible with those dates.
Once you are fully fledged you will be able to determine when people are in and schedule more in your own favour.
Would suggest that when you clean and people are in that you agree a date for the next clean there and then, put it on the back of a business card for them and make a note for yourself, could save a lot of hassle and confusion in the future.