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Your problem could be the battery, connections or you may have something drawing on the battery while the car is parked and everything should be powered down.
The first thing I'd recommend is having the battery load tested. It's hard to say if the dealer did this already but it's the first thing that should be done.
The battery connections should also be checked to make sure they are clean and tight. A corroded connection will cause enough voltage drop to prevent the starter from engaging.
If the battery tests ok and connections are clean then you have something drawing on the battery while the car is parked. The problem may be intermittent which prevented the dealer from finding the problem while they had it. What needs to be done is an ignition off draw test to check for a draw, if there is a draw then it can be traced in two ways. Voltage drop can be checked across fuses to see where current is flowing, or fuses can be removed one at a time to see when the draw goes away.
Garage says they tried all that you suggested but will try the draw test again. Thanks