Battery isolator not isolating properly

A battery isolator that's supposed to keep your starting battery separate from house batteries but isn't actually isolating (starting battery drains along with house batteries) usually means the isolator itself has failed in a way that leaves it permanently connected, or it was wired incorrectly to begin with. Solenoid-based isolators fail differently than diode-based ones, so knowing your isolator type narrows the diagnostic quite a bit. What type of isolator, and is this a new problem or has it always been this way?