Halion 3 .fxp instruments not loading in Halion 6

I have a large number of Halion 3 instruments that I still use from time to time. Most of them import into Halion 6 fine, but a few show up with the dreaded red circle with a bar in the middle to the left of the file name in the browser pane, like they’re an unsupported file type, but they aren’t. They’re just normal .fxp files and they used to run in Halion 3 just fine. If I drag one of these files into the slot rack, at first the instrument pops up like it’s going to load and then it just disappears. The ones that don’t import tend to be some of the more complicated instruments. Is there some feature that Halion 3 supported that Halion 6 doesn’t. Help… I can certainly provide examples.