Accessing soundfonts via Mixer

I've put a few soundfonts into the Soundfonts folder in Musescore4. When I open the mixer and try to access them, the Sound bar gives me a choice of MS Basic or Soundfonts. But when I try to open Soundfonts, there is a stall for a few seconds, the Soundfont list appears for about 1/100 of a second, and then it's gone. In effect, the Soundfont list is inactive, even though there are soundfonts in there. Any advice? Thanks.