Check for codes regarding the crankshaft and camshaft position sensors. When they start to go bad the ECU won't post a code until it happens twice in a row. In my case, it went slowly from an occasional refusal to crank (removing the key and re-cranking would work) to more often. For me it began cold weather, went away in the spring, and finally the sensor died in summer. Haven't had a starting problem since.
Dave