Make sure your clips overlap the lenght of the transition, one clip on 1A and the other on 1B, and that the direction arrow of the transition is in the right direction... from A to B or the reverse. Transitions as titles, usually set on track 2 or higher, can't be seen before you hit enter or select Preview. Make sure also that the blue bar on top of the timeline covers the area you want to render or preview.
Holding the <alt> while scrubbing the timelime will also permit you to see transitions but does not render them!
Hope this is clear!
;-)