pressure is critical
without enough pressure, the loco wont move itself up a slight grade, no less haul much.
i dont know what the recommended pressure is for you loco.
im a know nothing, only dabbling in live steam
id consider checking the jet to be certain it is clean, clear it by inserting a wire, (use tiny bit of wire) and properly adjusted to blend fuel and air
warm up and clear the cylinders as well, once you have pressure. i have learned to do this while the loco is raised on blocks above the track, and simply let the loco sputter and clear itself without a push, although a push works. i would think if you prefer to puch, pushing only in the direction of travel is best.
if you suspect the throttle doesn't open fully and is possibly interfering, perhaps disconnect the RC linkage, fire up , and manually open it up (within the parameters of your ROW, obviously wide open is fine on a flat, broad radius line, less so on uppy downy grades with sharp turns , etc.
i dont know the accucraft fire system, but, you might search the topic. sometimes, if the burner is a perforated stick type, one can wrap it in a fine stainless steel mesh with stainless steel band to better dissipate the flames and increase BTU dispersion under the boiler.
the other thing, actually, which i should have thought of first, is, is the fuel tank getting cold?
as gas leaves the fuel tank/container, the result is refrigeration, the low pressure results in the fuel getting cold and reducing the pressure into the burner. some fuel tanks have a surround , which hold water, and helps equalize and reduce this chilling effect. this can absolutely wreck pressure . but if you hold pressure without diminution over the period of the burn, this might not be relevant.