A reaction that can run both ways, A ⇌ B, never truly stops. It coasts to a dynamic equilibrium: A keeps turning into B and B keeps turning back into A, but the two rates have grown equal, so the visible amounts hold steady.
The plateau sits wherever the forward and reverse pushes balance. The ratio it settles on is the equilibrium constant K = k f / k r. Crank the forward dial and the balance slides toward B; it is not about running out of anything.
Shove in extra A and you tip the balance. The system answers by converting more A to B until the ratio [B]/[A] climbs back to K — Le Chatelier's principle: a system at equilibrium pushes back against whatever you do to it.
This is how the Haber process is tuned to feed the world: squeeze nitrogen and hydrogen harder and Le Chatelier hands you more ammonia.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.