Why German reinforcements could not easily restore the Balkans

Could Germany march divisions south and reverse an Allied breakthrough?

Only at heavy cost and delay; the route was contested and Ottoman defeat would free at least as many Allied troops.

Germany could not simply conjure ten divisions at the threatened point. They would have to move through Austria-Hungary and contested Balkan corridors while Russian, British, French, Italian, Greek, and formerly Ottoman-front forces concentrated against them. Removing those divisions from other fronts would deepen Germany's strategic problem. Reinforcement is therefore a choice among competing fronts and rail routes, not an instantaneous numerical answer to the Allied breakthrough.