Generator not picking up load automatically (auto-transfer switch)

An automatic transfer switch that doesn't switch to generator power when shore power drops (or vice versa) is either a switch that's failed, or a timing/sensing issue where the switch didn't correctly detect the power loss. Test the generator and shore power independently first to rule out either source being the actual problem before assuming the transfer switch itself is bad. Does the generator start on its own, or does that part fail too?