Change to 10/40 for winter and 20/50 for summer. If the temp drops to the minus in the night you will have problems starting. 20/50 is only good down to -10, although the temp might not get to -10 where I live in Hampshire it get down to -6 which is near the limits of 20/50. Might also be a battery problem.
I had the same problem as you then changed to 10/40 now starts first time what ever the temp gets down to. Make sure you change back to 20/50 about April time