Yamaha 4 stroke with automatic choke hard to start?

I have a 2 year old Yamaha 15 HP 4 stroke with an automatic choke. Previously, it always started right up. Lately, it's hard to start when first used, but then starts easily during the rest of the day. After starting, it runs fine. The fuel filter is clean, and I have recently cleaned the carburator (which actually was clean). I have also changed the spark plugs. Any suggestions?