I'm having the exact same problem, I could have beat this level numerous times but the fireflies won't clear as they should :( I Just figured it out read below
I figured it out... You have to match cropsies against bug three times but once lit you can't match another crospies next too it or it will close. So you need to focus on getting the bugs all lit at the same time and they will fly away. As long as a crospies match doesn't come along and screw you up on the interim.