Popups only being called intermittently

Popups for the same link only appear under certain circumstances. For example. If I click on "configure" on the blocks page in my own theme, the popup does not get called. If I flick to garland, and click the same link it does. At this stage I thought it must be something wrong with my theme. But then if I go back to my theme, and click the "configure" link again, the popup appears as advertised. I have no idea why this is happening, or what other information might be relevant to help solve it.