Here's a list I wrote awhile ago for a post on the newbie board and revised a bit for a blog entry on my website- since all four ladies have not responded, it may be something you said (or didn't say). Here's my list of tips for that initial contact:
So, you've found a few gals that pique your interest and are ready to start the scheduling process. Here are a few tips for that first email (or phone) contact to ensure that you make a good impression and get a date with your dream woman 
1. Read her site thoroughly (not just the pictures!) and be sure to provide all the screening info that is being asked for. We get so many emails; we don't have time to prod and probe you to get the most basic of screening information. Offer it up and you're more likely to get a prompt response and/or an appointment.
2. Be polite and friendly, but don't use specifics or words such as- oral, gfe, sex, etc. We know why you're emailing us, but putting it in these terms makes us suspicious. Be sure you have read her reviews and have a general idea of what she offers, but never assume that you will receive something and do not ask her to promise specific services in exchange for money- that is illegal. Pay attention to little things like grammar, punctuation, capitalization- these make a difference in that you come across as a professional and literate adult (Emails like "hey. iwant to see u wats the rate?" drive me absolutely crazy and do not warrant a response). As an independent provider who schedules her own appointments, I personally like a bit of a personal touch, a general introduction, etc. in the initial contact, but some ladies may prefer shorter or more business-like initial contacts, especially through an agency or booking service.
3. Suggest more than one day or time that might work, if available. She'll be much more likely to be able to accommodate you if you offer two or three possibilities. Also, she will know that you are serious and have planned ahead. If you don't have a specific date or time in mind, it's often a good idea to suggest days of the week and/or general times of day that tend to be best, and let her know how long you want to see her for as well- a dinner date requires different planning and scheduling than a one-hour appointment.
4. Contact her in advance if you can- I generally book 1-2 weeks out and feel that the sooner you contact her, the better. You are more likely to get your preferred time; she is more likely to have time to check references and feel comfortable with you. However, some ladies may not be able to commit to bookings more than a month or so out, so I would advise 1-3 weeks in advance as the ideal time frame (closer to one week for shorter dates, and more like two to three weeks for longer encounters).
5. Pay attention to how she prefers to be contacted, whether agency or independent. Don't waste your time emailing her if she operates by phone, or vice versa. If she has a contact form, fill out ALL the required fields. In addition, if a woman is represented by an agency or a secretary, pay attention to their requirements for contact and screening.
6. Don't assume that because you've contacted her or filled out a form that you have made an appointment. You need to hear back from her to confirm it. Generally there will be several emails back and forth before a date and time is settled upon, and she will also need time to check your screening info, so be patient. That said, if you wait more than 3-4 days for an initial response, it's not unheard of to send a polite second email asking her if she received your initial one. However, don't email her every day or demand a response within a specific time frame. If you feel she is taking too long to respond, move on to someone else and maybe try her again down the road if you are still interested. In my opinion, checking and responding to my business email account is a huge part of my job, and if a lady consistently ignores business emails from clients (given that they are polite and provide all the requisite information), it can be a sign of unprofessional conduct or unreliability. On the other hand, if you don't hear back, don't assume the worst- she could be dealing with a health or family issue or out of town on a personal or work-related trip.
7. Once your appointment is scheduled, sit back and enjoy the delicious anticipation!! If you schedule more than a day or two in advance (as you hopefully will), most ladies will contact you by email or phone to confirm on the day before or the day of the appointment. Every lady does this differently, so ask her if and how she will confirm with you or check her website and reviews for such details. However, don't expect her to email or call you daily or regularly in between setting the appointment and meeting in person. Just like your therapist or your personal trainer, we are professionals- we set appointments and we keep them, and expect you will to, without constant reminders and updates. However, as stated, it is generally common courtesy to confirm and check in with you on the day before or day of the date, and if you don't hear from her or she doesn't do confirmations, you can feel free to email or call her (whatever her preferred means of communication) to say hello and confirm the date yourself.
Hope some of you newer and seasoned hobbyists alike find this helpful and informative, and best of luck scoring that hot date :-P