Mine never starts after oil change either but that is because I also clean the air filter with soap and water before putting on the filter oil..
I figure it is the residual water on the air filter that is the problem, because when i let it dry out for a couple of hours it starts right up fine..
Not to be Mr. Obvious here...I would never install a filter that has any residual of water on it. You need to completely let it dry and oil it thouroughly before re-installing. Buy an extra filter and have it ready to go & oiled in a plastic ziplock bag - to avoid having to wait for the dirty one to dry.
Changing the Oil should have no effect on starting the bike. I usually go about a year between plug changes and only change the plug as a preventative maintenance measure. I could see with the weather getting colder in most areas, starting/plug fouling problems will increase due to the wrong jetting for the temp.