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The 110V outlets on the aircraft have a safety feature. You have to plug in the item. The prongs slide in and trigger a switch that then gets the juice going. So, sometimes you'll see a small (1 second or two) time delay from when you plug things in, until the juice starts flowing. Make sure the prongs on your Dell's plug are not damaged.
Additionally, the outlets are rated to a maximum output wattage. Some of the larger laptops have A/C adapters that pull up to 100 watts. My co-worker's 17" HP has a 90W power adapter, so it will definitely not charge (while running) on an aircraft. IIRC the PMCO aircraft are rated at 75W.
Most 15" laptops have 65W adapters. Check the adapter label.