What is the difference between the Montagues' and the Capulets' ability to make peace?

I'm not sure exactly what you're asking, but my guess is you're inquiring about the Montagues' and the Capulets' inability to work things out before their children committed suicide.  Peace only came to their families after they realized the price it cost them.  Romeo and Juliet had to move around in secrecy based on the fact that their families had been fueling a terrible argument for centuries.  After the funeral of thier children, they changed their tune.  A little too late, but they changed nonetheless.