Troubles applying a mending plate?

It keeps bending/bouncing/deflecting because it is no longer structurally sound. It needs to either be replaced or you need to reinforce it by nailing the same size piece of lumber onto it. Simply measure the existing beam and cut a piece to those measurements, then nail it into place. This will fix the immediate problem by transferring the load to the new beam. After you do this have a licensed home inspector or a licensed contractor come look at your roof to make sure there isn't a larger problem that caused the beam to split in the first place.