I'm using P4 and have found that the transitions do not run smoothly. For example, when one clip is ending the new clip (e.g. a child running) doesn't start until the transition is over. Used dissolve, wipe and both transitions had the issue. When the clips are butted next to each other w/o a transition there is no issue.
In another example the dissolve artificated and discolored the first two frames of the 2nd clip.
OK, reason probably is then that you did not trim half a second from the end and beginning of the clips (assuming you are using a second transition). Otherwise with the transition properties panel open drag the scond clip so it overlaps the first.