I am managing a website called Rent A Boat Amsterdam. We have a system that gathers reviews from people that have used our services and that publishes these customer reviews making them available for all website visitors.

When these customer reviews are published we have placed them within the appropriate tags according to the guidelines set by Google, which you can find here.

An example looks like this:

<li class="" style="clear:both;"> 
<div class="hreview"> 
    <div class="item" style="display:none;"><span class="fn">Boatname</span></div> 
    <div style="border:1px solid #DEDEDE; background-color:#D9FFD4; margin:0 10px 10px 0; float:left; text-align:center; padding:10px; height:50px; width:70px;"><h1><span class="rating">10</span></h1>9-Jun-2010</div>                    
<div> 
<div class="description"><p>Great canal Cruise!</p></div> 
<p class="reviewer vcard"><strong><span class="fn">First name Last name</span></strong></p> 
</div> 
</div> 

We have implemented these tags a couple of months ago, but there are no visible results in the Google SERP's. This whilst I had expected to find the reviews / ratings displayed similar to:

Is anyone familiar with this topic and able to help me find the answer to the question why the review-aggregate format doesn't seem to have the desired effect?