With a couple fingers holding the plant in place, and other fingers firmly on the surface of the media, firmly tap the pot on the ground/counter/whatever surface you are using - several times. Also try firmly tapping the sides of pot with one hand, while you hold the pot in the other. This should help the media settle better, and you may find you can add more media and repeat the process.
Some plants will be a bit shaky in pots for a bit after repotting, til they get established in the pot - but in most cases, I can get plants quite secure in their pots by repeating the tapping down of the media.
If the plant has little, or no roots, it may not be possible to get them very secure in the pot. Try to avoid disturbing/jostling the plant, so it can hopefully get itself better established. Some people use clips to hold plants secure, or devise their own ways to accomplish it - stakes and wire.
Hope this helps some - gl