Good advice below already.
Just a couple of quick observations.
First, I've found that no matter where I advertise, people ask the same questions that are already answered in the ad. Interestingly, though, my worst experiences along those lines was years ago when I was advertising in the Washington Post. Some questions over and over, all answered in the ads. The price. The location. Number of bedrooms. And so on. I couldn't (and still can't) figure out why people did that. Maybe they're trying to confirm the information. Maybe they've written down phone numbers from the ad, but don't have the ad in front of them. I don't know. It's a mystery.
I'll admit, though, that it's gotten better. I've used Craigslist and VFlyer, among others, and the questions I've gotten recently are pretty specific and often not answered in the ad. Or people weren't familiar with a concept (such as ground rent for a manufactured home), even though I tried to explain it in the ad.
Also, many renters are very flaky. Many change their minds from day to day about what they want, where they want to live, and so on. So they'll e-mail and you'll answer. In the intervening few hours, they've decided they want to live on the North side of town, not the South. Or they've broken up with their boyfriend or girlfriend. Or their mom or dad has advised them to do something else. The only thing to do is to follow up with them regularly--say once a week. If you have their e-mail address, e-mail them once a week, cordially, and let them know that the property is available, and remind them which one yours is. I've also found that sometimes people will call 20-30 different places and have no idea who they've called or e-mailed.
Very good advice from Cheryl and Eric, especially. If you aren't getting the quality of leads you're paying for, then stop using the service. It might work for others, but seems not to be working for you. And just double-check your rental price to make sure it's competitive.
But, overall, your experience isn't all that unusual.
Hope that helps.