AM adds an onload event that translates the page when it's initially loaded. On ajax requests, you have to manually trigger the re-render. Do this by calling the AMprocessNode function, feeding it the element that contains the new text to be rendered. For example,
targetel = document.getElementById(target);
targetel.innerHTML = req.responseText;
AMprocessNode(targetel);
The process is different if you're using MathJax, but since you posted here, I'm assuming you're using the classic AsciiMathML script.
David