I too have been having trouble with the neck of the sediment bowl getting plugged up when everything else looks fine. I think the ethanol or some other additive is reacting with the aluminum in the bowl housing. 

If you have good flow at the carburetor then it could be a plugged or sticky needle valve or a sticking float. The next thing would be a piece of crud in the load jet. I have had this happen, and while the tractor is running turn the load screw out a half turn for a moment and then turn it back in and have whatever crud was in there come out.