I was having the same issue, and the fuel boost pumps were the issue.
The checklists in the manual are confusing...you cant use the 'engine startup' checklist just by itself, as the 'pre-flight' switch check before that has stuff in it that affect how the aircraft starts up...these being the prime case. (Edit: meaning you turn the fuel boost pumps on prior to any of the stuff in the startup checklist)
The training mission on engine startup is great, and I made myself my own checklist based on that (since we're in a sim, I don't need to check the position of every switch before I start the plane...I'd imagine the planes have a standard set of initial conditions when they 'spawn')