Multiplus 12v/3000/120 not charging batteries from generator

it seems intermittent, but recently multiplus doesn’t switch to charge mode when generator switched on - has happened twice now. No apparent loose wires/connections. On the Color control pages - you can see the incoming voltage and frequency from the generator but charger won’t switch to charge mode. I have tried a different generator - same issue. Without doing anything the system then started to charge.