It's been suggested that permeability declines with the age of rubber, so maybe your tubes are just getting old. But the downside to that is it gets brittle so that's probably not it
Yet I was just thinking about it last week, and it seems to me that a patch is going to be more air-tight than the bare tube, being thicker and with the vulcanizing. So the more the tube is patched, the better it will hold air - eventually my tube will be covered entirely and I might never have to add air.