What determines how many notes you must hit to increase the multiplier?

[Magnet]

Foreplay/Longtime on the PS3. I think after the solo there are like 2 or 3 Vocal gems and you can't increase the multiplier

I just played this song on the PS3 and was able to increase the vocal level every time the vocal track was active. Not sure what you're talking about.

[Bront20]

I just played this song on the PS3 and was able to increase the vocal level every time the vocal track was active. Not sure what you're talking about.

I must be missing the notes then.