Invisalign trays are too loose, they fall out
Not heard this one before.
Invisalign works by the concept that each aligner is actually the next step in the orthodontic sequence to the tooth in question. This anomaly means that the aligner is actually tight.
Over a period of 2 weeks the tooth moves and the aligner loosens.
Each tooth has an undercut around which the aligner fits, so they should not fall out.
It seems like you may have ill fitting aligners.