There are multiple touch points and questions to ask to answer your question. I will try to give an high level overview, but there could be one to several "breaks"  in the chain.

1: what you are ranking for, there are keywords, keyword counts and  "buyer" keywords take a look at this sample: FIRST column is keyword and second column is monthly searches

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So, looking at this example if you rank for keywords with low counts in the searches then you need a lot of them to equal a good amount of searches to reach a volume level to have a good flow of leads, also you have keywords that are from a lot of "tire kickers", people that are just looking for information and "buyer" keywords, people that need your services NOW. That could be a huge difference...

2: The second issues could be your lead handling. Such things as 
      a: Voice mails, time to answer, call back timing, how you answer the phone
      b: your quote handling, professional looking quotes
      c: billing, ease of pay, etc

3: Trust Factor, do you have enough of the right kind of recommendations?
4: How do you look? your trucks, equipment, personal professionalism? 

These are just a few of some of the many things that could be missing. Also, how aggressive are you pursuing contracts with companies that could provide a steady flow of business? You have to be extra sharp to land those type contracts and be prepared to give customized client processing, like billing, call handling, etc.


Hope this helps some....

JB
LeadValets