Many people only try it for a couple months then give up. Or they run 1-2 ads a day, you need to do at least 4.
^I can't stress how vital this guy's posts have been. Seriously, people pay for advice like this.
A while back a partner and I experimented with radio. We found that the 5-6 time slot is stupid competitive and most listeners on music radio switch during commercials. Our best return was with church/religious stations, talk and sports radio.
The other businesses advertising at the stations that were reeling in clients/customers were the ones who'd been advertising for 2+ years and had a unique sponsored segment. For instance, A doctor had a 10 minute on air Q&A segment, a home builder had a 60 second home improvement tip, etc. Some on the sports radio just had the business owner call in and shoot the bull for 30-60 seconds. All essentially created an on-air personality.