If it uses the McDanial style, they can be a real bear some times. I have had them break on YS 1.10's because you could not grip it good enough to twist it hard engough (not enough clearance to grip it with a needle nose) and the wire would pull out. This is the solution I have found, and it works like no other.
It is the connector that most pattern flyers use, and it iwll never vibrate loose, or have a bad contact, and it is easy to remove. It is machined out of brass to fit OS and YS plugs.
The link that I provided shows the one I am mentioning, and the other styles that you can maybe use to identify yours.
http://www.centralhobbies.com/glow/headlock.htm