Put the frag on a plug with no flow for about 24hrs, you can dab a bit of glue to help it along but you aren't really gluing it to the plug you lightly attaching it while it attaches itself. Generally gluing takes some practice, but I'd say about 50% or more of the time I can glue one to a plug and it will stay.
Other option would be to put the loose frag in a pile of rubble. Again no flow at all for as long as you can. You can put them in a water bottle with a few tiny holes so there is the smallest bit of circulation. A small container in tank works as well. You still need to keep the water warm and want a tiny bit of recirculation but even a tiny bit of flow will knock them off.