I just switched to premiere from Vegas Pro 12, so I'm still unfamiliar with premiere. For some reason, one of my clips has grayed out corners on the timeline and won't accept transitions between itself and the clip before it. I have no idea why the clip is acting differently, and couldn't find what I was looking at on the transitions help thing, other than knowing WHAT is going wrong. no idea why or how to fix it.
The gray corner is an indication that you've reached the very end of the clip, no more frames. For a proper centered transition, you need frames after the end of the clip in the sequence. So cut back your end point by half the duration of your transition.