Stacks of images won't show top (first) image of the series consistently.

I often create stacks from Bracketed sets of photos I shoot doing real estate, so I can compile them into HDR. When sorting through the stacks, it won't consistently show the first shot taken as the main thumbnail. It will often cycle to other images in the stack, some of which are very dark exposures from the bracket. This is very difficult when trying to sort through the stacks to select the images I want.