I find that with a lot of extruded tubing the internal dimensions can be rather variable. Either out of round, lumps and bumps etc.
I find the larger the sizes are more prone to this, as the tolerance is generally a percentage of the tubing diameter.
It does depend on the manufacturer/supplier, some supply better stuff than others.
Perhaps it's for this reason that most of the ready made WTC's of larger diameter use axial compression of the seals instead of radial compression.
If radial compression is a must, a larger and more compliant o-ring may help, or simply doubling up of o-rings. Other solutions include machining a collar to be bonded in to the tube, or even machining out the tube itself, if you have lathe large enough and sufficient material to enable that course of action.