I'm not sure of the pin size, perhaps attach some images of the board and devices you are removing. Pace also sells thermodrive tips which have more mass up front than the standard desoldering tips.
If the thermal dissipation is great enough it can make using any tool challenging no matter what the power. Using a preheater sounds like a solution, it greatly assists all desoldering. A hot air gun alone also tends to heat locally and a lot of the energy is deflected or dispersed. With preheating to allow a greater area to slowly rise, less energy is required from the desoldering tip to bring the target area to melt temp.
An alternative is to try slowly bringing the temp up on a variable temp hot air station with the tool mounted in a bracket on an angle or with a broad angled nozzle, allow plenty of time for the heat to soak in. Then use the desoldering tool to come straight in. Or even apply heat from both sides of the PCB if possible with hot air and preheating before desoldering.
Ensure there is good contact with the desoldering tip to the pad and a thermal bridge is created. If an oversized tip is used it may miss the pad and heating performance will be reduced. If it's a huge cold ground plane it will likely drag the tip temp down so a high or higher desoldering temperature may also be required, obviously you have to be cautious to prevent PCB damage at the same time.
As suggested you could also apply some low temp solder or even leaded solder and flux and freshen the joints, this aids in desoldering. If you get complete melt you also may need to wiggle the pins a bit while vacuuming to break the surface tension around the entire pin.
Another trick is to use a solder bath or a custom made desoldering plate.