I do have the same (similar) issue: I have the P3d standard default Scenario (with the F-22), but do start my flight with the DC-8 preselected through standard menu: I set DC-8 aircraft and pick Airport / parking spot. Sometimes I do set up a flight plan already before starting the scenario, but as for the DC-8 I usually set flight plan while I'm at the ramp only.
Anyway, after starting the scenario, the DC-8 sits at the parking with engines running, then I select "cold&dark" from Shift-3 menu (usually after I switched through all views).
I follow the steps given by the checklist / tutorial flight. When it comes to engine starting, I have the ingnition armed, external power and air connected. I set the fuel levers to off and main boost pump #3 to boost&feed.
When I hit the starter button #3, it spools up and I select fuel lever on, but it is not getting "over the hump". So I cancel by depressing starter button #3.
Then I can't do a second try with #3, as it just won't spool up anymore. I can try the other engines though, but same thing: N2 RPM goes to around "2", then nothing.
I found a cumbersome workaround for me: I hit CTRL+E to have the Sim doing the default start procedure. Don't get me wrong, I'm not expecting this to work properly, but at least it helps like this:
After CTRL+E, #1 spools up, but still no ignition. Default procedure then stops automatically. Now I'm able to start #1 as per the checklist.
I have to repeat this for each engine, so first CTRL+E, wait until it fails and Sim stops the default procedure (N2 Drops again), then do normal start up procedure. Well..