QuickLOAD didn't like the data. It wanted to give me about 1285 fps and 35,600 psi. I had to change the case volume way too much to get down to your velocity, so I switched to a more simple means of estimating using QuickLOAD's muzzle pressure number as real and got an expected peak pressure of about 22,000 psi.
The discrepancy may be due to the roughly 62% case fill you have. If the powder is forward in the case (up against the bullet) do you get a significantly different velocity than with the powder over the primer? I'm not sure how much position sensitivity to expect from 2400 with such a heavy bullets. I'd have thought, not much. Another sign of an ignition issue would be a lot of unburned powder. If you put a white piece of paper on the ground a few feet out in front of the firing point, does it collect a lot of unburned flakes? If so incomplete combustion could be the issue.
If you can do it, take a .22 rimfire rifle and fire some match ammo over the chronograph to verify its accuracy. That usually comes within 50 fps of what is published on the box when fired from a rifle. That's because of the huge expansion ratio and the fact the bullet stops accelerating somewhere between 16 and 19 inches down the tube, then doesn't lose velocity very quickly with additional bore. That makes the exact barrel length non-critical.
If you have the chronograph over light colored soil, I would lay some black plastic or flat black cardboard on the ground in front of the instrument to prevent light glinting off the bullet, which can cause early triggering of the front screen. I might even go for a supplementary diffuser if the sunlight is direct.
This is all a matter making sure of the readings. I would also move the chronograph back to 20 feet. I don't think you should see muzzle blast effects at 15 feet with this charge weight, but if a lot of flakes are blowing out ahead of the bullet, then that's another possible source of early triggering of the front screen. Just don't shoot the machine.