I know amplitude doesn't depend on mass but I came across a problem where the amplitude changed with a change in mass. The problem was a horizontal frictionless spring with a box of mass m1 attached to it with a stone of mass m2 inside of the box. At its equilibrium point at its maximum speed, the stone left the box and the amplitude got smaller. I'm confused as to why it changed? I can post the problem if it's not enough information.