Eastwood Flaring Tool Issue with Cupro-Nickel

It's too soft for those tools, I've had similar issues. I ended up using compression fittings when I tried to use it.
As dubob said, shorten the amount of line hanging out, see if it improves. That will reduce the tendency to buckle.
The upside is, because it is softer, it doesn't have to be perfect, it will conform better than steel.