Normally it's when I don't drive it for a few days. If you let the pump prime, then let the car sit for a while (30 seconds) with the key in the "on" position, you will find the car fires a lot quicker! I believe this is due to fuel having to displace the ambient pressure in the lines.
I have an AEM ems, which primes a little longer than normal, so that led me to that conclusion!