I have had similar problems to what you're describing, but not specifically with IVM (or any other NCH software for that matter).
I know I have had SAPI 5 voices that simply refused to play in some TTS software, but worked fine in others (it always worked with IVM for me though).
I also had some SAPI 5 voices that I couldn't get to work at all with most TTS software, after re-installing them the problem seemed to be corrected.
Lastly, I've heard that some TTS voices require that you have certain licensed software installed and/or running (e.g. Text Aloud) otherwise the Voices will refuse to work.
My Suggestions:
- Restart the computer if you haven't already done so
- Re-install the Voice & related software if possible
- Try using the Voice in other TTS programs like TextAloud
- Contact the manufacturers/creators of the voice and see if there are any special requirements for using the voice in third party apps.
By the way, what TTS voice are you using?