Sig p226 firing issues

Hi all.i have a p226 in 40s&w and I love it. I'm having my first issue with it though after 5 years of no problems: when I have a round in the chamber and the hammer decockeda I am unable to discharge said round in the chamber. Now if I manually cock the hammer back on the same said round it will fire no problem. Any ideas what could be causing this? I have used about 5-8 different types of rounds and it happens regardless of the maker. Also I have cleaned the gun as prescribed in the owners manual and still the same results.