Most girls don't allow it because they are afraid your hands are too rough and you scratch or cut them with your finger nails, so make shore they are well manicured,
also a girl has to work long hours so she most likely doesn't want to orgasm until end of shift, plus you need to ask the girl personally if you are allow to touch them there and pay them the right tip eg 2 yellows for part3, one girl might agree to do it with one client because she trust him as a regular, but won't trust a new client,
so go find yourself a regular WL and your chances may improve