How this is done: In the Action pannel, select the 'infinite' checkbox. There are "On Success" and "On Failure" drop downs below this. I would advise that you save these settings IMMEDIATELY after making your interaction.
I have on many occasions set these to do what I would like, do some other work on the project, (set the size that images will shrink down to once dropped in the appropriate area) and these options will become inactive. I am not sure why this kept happening, so I called adobe support and had a screen share session. He couldn't find out why this was happening either, and every time it did I would just delete the interaction and start again.
So keep checking the action pannel after every move you make and save your work frequently to avoid running into this extremely frustrating bug.