Are you referring to custom animation, or Slide Transition? Take note that
they are not the same.

For slide transition, click on the transition you want, but do NOT apply to
all slides. If you have done so, remove it by selecting No transition, then
click apply to all slides. Now, go to the slide that you want to add the
transition on, and select the transition.

Hi Jennifer

If you're talking about movies or animated gifs or whatever, you need to go
to the "effect" dialogue (which you'll find under "custom animation",  The
middle section is "stop playing" with three radio buttons.  The one you want
is "after current slide".

Hope this helps.

Fiona