Which cultivar...some do fall off when near ripe but should soften up in the house in 2 to 3 days...if that doesn't do it you might have a dud lol.

It's a Tecumseh plum

I don't have this one...was in very windy, sometime that can knock things down unripened.

The winds been ok, nothing too crazy. But I'll try ripen them in the house here, thanks

In my experience every plum has its time to pick. Some like euro plums you can let hang for a long time and they will still hold form once picked for weeks.

Some jap ones you have to pick them when they are just turning a bit soft cause if you don't you'll have mushy plums that are hard to use for anything.

it takes a few years of experimenting with your fruits to figure it out.