Originally Posted by FBinNY

Odds are that inside the freehub section of the hub, the ratchet area is packed with dried grease or has some rust. Either could be enough to keep the light springs from engage the ratchet solidly when you switch from coasting to pedaling.

Often if the situation isn't too far gone, the unit can be saved with a solvent flush and relubrication. If it's badly rusted (inside) it calls for a replacement of the freehub module. If you prefer to do your own service, search "servicing freehubs" on the internet for some tutorials.