Did you oil the filter when you cleaned it?... if you did you might have used too much oil and its doing what Jon said.
You just want a thin film of oil on it.
Also if you have points and they get dirty the spark get's weaker and won't fire the plug.
I think the quickest way to find out is to buy a new plug and see if it works good again.
I bet 70% of the ignition coils ever bought for any vehicle were not necessary because it's usually something else.