Interesting. I have a similar problem I solved using AddFontResourceEx(), but it was sort of the reverse. I have a library using FreeType that wouldn't 'see' fonts Adobe was loading (because they were coming from their own folders and not the Windows Font folder).

You have the reverse problem, sort of. You also need to load additional fonts from (presumably) odd locations, but you want Illustrator to see the result. I'm not sure what to do for your case, though perhaps its a matter of timing? Are you seeing this "sometimes it works, sometimes it doesn't" on the same machine, or different ones? If the latter, it could just be the plugin load order. If it's the same machine, that's more troubling.

Either way, your best bet is to find as early a place as possible to do the font loading. Where are you doing it now?

Is there any reason you can't pre-load the fonts before they're clicked on in the panel? Or is that the point of the plugin, to load additional fonts?

I don't know what causes the delay, or if there's a way to 'kick' it to make it update faster. The best bet is to have done it long before its needed so they're already there, but if that's the point of the panel, I guess we'll need to see if we can find something else.

I don't see anything that lets you kick the font listing. You're cleary trying something along those lines with SendNotifyMessage(). I suspect it treats that as a call from a different thread, so sometimes it processes it before you count the fonts and sometimes it doesn't.

Have you tried SendMessageCallback()? That lets you do the same thing (I think, more or less) but SendMessageCallback should invoke a callback function when the message has been processed. You could try that, and see if you get consistent results inside the callback function.

Otherwise, I don't have any idae. This is strange territory for Illustrator, and its starting to sound like its an OS problem more than an application problem (if only because the SDK doesn't provide any help).