Turns over but will not start

Club Cadet, Briggs and Stratton 22 HP twin OHV.

Will crank fine, but not firing up. Mowed great all season, last time it was used it took about 5-8 seconds to fire up (a little longer then usual). Now it will turn over for 15-20 seconds, multiple times over several days, but will not start.

I have dismantled the top cover and checked for any loose wiring.
Have removed one of the spark plugs to check for firing and its working fine.
Also have removed fuel line at the carburetor intake and there is flowing gas.

Any suggestions?

Fuel, spark, compression, and timing and it will run. Start small like spark plug and filters.

I've had some B&S twins come in with the same symptom and it turned out that the choke was not closing all the way due to an out-of-adjustment choke cable. To properly set choke cable adjustment have a helper hold the choke in the fully applied position, loosen the cable housing pinch bolt, gently pull out on the housing and tighten the pinch bolt. Make sure you are working with the choke cable as there may also be a separate throttle cable. I'm not saying that is what's wrong in your case, just something to check and make sure the choke is closing all the way.