That's because in order for any dissolve to happen, clips need to overlap.
Premiere uses extra frames called "handle" frames outside of your cut to make this possible. If your clips are of exact captured lengh, then those extra frames are not available.
Your options are to either shorten the clips to allow dissolve time or to overlap the clips on 2 tracks and apply a dissolve to the end of the top clip and the beginning of the bottom clip.
Vince Becquiot
Kaptis Studios
San Francisco - Bay Area