Sometimes, Video Transitions doesn't get added

Sometimes, I try to add Blur Dissolve or Cross Dissolve but they just don't add to the clip. I drag them and try to add them to the start or end of the clip but they don't show the box.
Other times it works just perfect. Can someone please tell if this is a bug or something else?

You will only get cross disolves if there is enough video to mix out from one clip into the other. If you have the very end of one clip butted up to the very start of the next clip and neither have extra footage, you have nothing to work with at the cross point.