There is not enough information in your post to properly analyze your situation. For example, you have not given the version of MathJax you are using, or your configuration, or what browser you are using, or what OS you are running on. All of those might be important. Finally, you should provide the MathML (or TeX) expression that you are using so that we can test the results ourselves to see what is going on. A screen shot is useful, but not enough to be able to check what is actually happening.
It looks like you might be using SVG output. You might try switching to the CHTML output renderer instead and see if that helps.
Davide