I installed stick-down tiles over my (vinyl) bathroom floor.
In a few weeks, many of the tiles had lifted, or moved.
Someone in this group suggested varnish as a "sealer"
I removed the loose tiles, ( yes, the floor was clean )
and used spray varnish on the vinyl. ( it's what I had on hand )
Re-installed the stick-downs, and, after several weeks,
it looks like those tiles aren't going anywhere.
Thanks to whoever posted the tip.
I'd definitely suggest a coat of varnish over the old floor
before installing stick-down tiles. It worked for me !!