Any coolant leak will admit air into the system and stop the coolant recovery system from working as will a defective radiator cap, an air leak in the hose to the recovery tank or a leak in the tank itself.
If you have a good radiator shop in your area, they may be better at diagnosing this than the dealer.
A member of our church had donated a Dodge minivan to our minister and the minister was having trouble with it overheating. The donor was unavailable to ask anything but the repair receipts where in the van. The radiator had been replaced twice, and “rodded” once the thermostat had been replaced three times and the cooling system had been flushed. This had all been done at the dealer.
I drove the car and confirmed that it was overheating, and in a peculiar manner. It only overheated between 45 and 55 mph. it cooled down while idling or at expressway speed. When I stopped the car and walked to the front the heat just blasted you so there was no doubt it was overheating, I was stumped.
The minister took it to a local independent garage where they spotted the problem almost instantly, by noticing the clue that the dealer and I had seen but overlooked. The heat was blasting out from the front of the car.
The electric radiator fans were running backwards, pushing air forward through the radiator! That air would cool the car at idle and low speed but between 45-55 mph the air trying to flow in and the air being pushed out reached a stalemate.
When the previous owner came back that the van had been in a left front collision and the radiator, battery fender and grill were replaced and some wiring repaired. Whoever fixed the wiring had cross connected the fan wiring. Countless trips to the dealer had failed to spot the problem they had caused.