If the car has already been brought up to temp, then generally they don't prime, this has nothing at all to do with the pressure already in the fuel rail/line, as there is no pressure sensor the ecu can't know what pressure there is, it is a time/temperature based decision whether to prime or not, and each car can be very different.
Sounds more like a loose connection, just do the wiggle test with the engine running, better to sort now than pray when in the outside lane of the M25 when you remember what you forgot to look at!!
tabetha