Several reasons. As someone said Transfer switch stuck, Breaker (on generator) OFF, my favorite is "Screw Loose" or in this case a connection (The screws are the ones that hold the wires)
And in this special case Generator out of spec.. The inverter monitors 2 or 3 pramaters, Voltage, Frequency and optionally waveform.. Plug a Kill-a-watt into a NON INVERTER outlet and check the voltage and frequency. May be out of spec.
Waveform takes an electronics technicican with a 'scope.. Odds are though if your Generator is an Onan or Generac RV type generagor it's good.