Check the large fuel vapor hose that leads from the regulator to the mixer for any signs of splitting or unraveling. Any kind of breach in this hose can result in the fuel hose and mixer being purged completely of fuel while the engine is turned off more than a few minutes.
Also, I have had cases where the main fuel solenoid (big one) rubber valve face seems to stick shut tight enough that a weak battery might not jolt it strong enough to open it during a "cold cranking event" where current draw from the starter load saps the voltage in the system. So a good hot battery is important. You can lightly tap on the main fuel solenoid with a plastic screwdriver handle or similar object when you first try a cold crank. If tapping cause the engine to come alive your problem may be a marginal battery.