Why does my function only run once per page load?

I'm writing a script that does a calculation with text inputs in a form. For some reason, my calculation is performed only once per page load. I have to refresh the page to get the function to run again when I press a "calculate" button.

Here's the HTML for my buttons:

<input type="button" value="Calculate" onclick="calculate(high, low, common);" />
<input type="reset" />
<div id="result"></div> 

Here is the calculate function (the variables are defined elsewhere):

var calculate = function (high, low, common) {
   if (high.length < 1 || low.length < 1 || common.length <1) {
       document.getElementById('result').innerHTML="Please enter a number for all fields.";
   } else {
       if ((high - common) > (common - low)) {
           document.getElementById('result').innerHTML="Result 1.";
        } else if ((common - low) > (high - common)) {
        document.getElementById('result').innerHTML="Result 2.";
        } else if ((common - low) === (high - common)) {
        document.getElementById('result').innerHTML="Result 3.";
        }
      }
   }; 

Am I doing something that prevents the function from running more than once per page load?