I import a flight plan made my Simbrief. I validate and file. It gives a departure runway, SID and arrival runway, but no STAR. When I add a STAR, I have to validate and file again. it then deletes my STAR from the plan! I'm probably doing something stupid!

There is a known issue with star's that Dave is aware and promised to fixed in the next update.

If you disable the star option in the config flight plan tab, this should allow you to set the star yourself and use autoplan which won't remove.