More powder,same velocity

[denton]

I think that the opinions posted are quite correct, but there is one other possibility that needs to be considered.

If you take run-of-the mill brass, fill it precisely with identical amounts of powder, and use unselected bullets, you will still get fairly significant random variation in muzzle speed. The typical standard deviation is perhaps 30 fps, which is about 1%, which means that 95% of all shots will be within 60 fps, which is about 2%.

If your powder is going up in 2% steps, your muzzle speed is only going up in about 1% steps, and that is well within the normal, random error of the process. A single cartridge in such a series, that fails to show an increase in speed, means next to nothing.

If the pattern continues to show itself, with several shots, then, indeed, you're at saturation, and more fuel won't make the bullet go faster. This can also be detected by the presence of large wood splinters in your left forearm.