My experience with a Gardenline chainsaw (and they're all pretty much the same) is that you have to spin the engine quite fast to generate a spark. Holding the plug against the cylinder head and pulling the cord isn't good enough: you have to pull it hard to spin the engine.
I replaced the coil before I realised this - and found there was nothing wrong with the original!
What is also very important is the gap between the coil unit and the magnet in the engine flywheel. It should be about 15 thou (0.015" or 0.4mm)
David, August 2009