I've had brand new ones do this to me as well. I normally don't leave
brushes sitting in water (one cause); I try to get them cleaned immediately
at the end of the lesson. But even the heavy duty washing seems to loosen
up the glue and then they become wobbly or fall off.
I don't have a recommendation for a particular brush -- even my personal
superior quality watercolor brushes loosen up after time. What I do for
my classroom brushes that come apart is to use a bonding type glue, like
527. A couple of drops in the ferrule extends the life tremendously. Like
Hope this helps.