Sometimes it really is just all about timing. If you have a lot of daycares in your area then you may not get a lot of bites.
There are also periods of time where I don't hear any emails or calls for months. I had nothing from Nov-March and then suddenly people are calling again. Finally after nearly 2 years of being open it was my first time where I had two people interested in the same spot. I thought that sort of situation was the stuff of myth and legends judging by how slow it was going.
The other thing I have during slow periods is odd people coming where I know that they are just not a good fit for my daycare. Unfortunately it takes a lot of time and patience to find good people. I don't think you are doing anything wrong.
Sometimes a picture of your play area might attract more people through daycare bear....or a picture of your backyard if you have nice toys back there.
The only suggestion I have is that your ad currently says
"I am looking to fill a full time spot for April 23rd, 2012"
That may be too specific. What about a parent who is looking for June or July?
Maybe word it a bit different and vaguely --like "We are looking for a new little friend to join our group starting in May or after".
Also...under your location you may want to remove your actual house number. Just give them the street name and city to start just for your own security. if they contact you for an interview then you can give them your house number.
Good luck. think positive thoughts and keep on trying and they will come