Won't start off main battery

[Electric Mayhem]

Hi my NT is having trouble starting of a morning. It turns over, power to everything but won't start. It will start off aux battery first time. After a decent drive it starts off main battery.

Something seems to be draining the main battery. Nothing is left on in the car. There is an isolator between the batteries?

Any clues before I take it to an auto sparky?

Thanks

[spot01]

How old is the battery, what brand/type/size?
Have you cleaned the terminals (even clean looking terminals sometimes need a removal & clean)?

Have you measured it's voltage at rest & running?