Forgive my ignorance if this suggestion has no basis in reality as I know next to nothing about these things,
but I thought that it might be possible.
Are your bullets touching the lands at the OAL you are using? If so, that in itself would prevent the round from being forced forward by the primer strike, regardless of case/shoulder length.
If the case is shorter but the OAL is the same then the bullet must be farther out of the case. Farther out means lower pressure, right? Which would effect the "time-in-barrel". Closer to an optimum muzzle exit time?....
http://www.the-long-family.com/OBT_paper.htm