You need to evaluate what has changed between when you identified your element, and when you try to use it later. I recommend re-spying it into a new element and comparing the two. There are a number of elements that are prone to change on some web pages (ID, URL, even path sometimes).
The other issue that could be occuring is that you are running at full spead and your element is not loaded at the point when you attempt to use it. I recommend using the 'Parent Document Loaded' option in wait stages at the start of all HTML actions.
Also, if you have problems with the HTML inteface for a website, the AA mode usually works as an alternative.