So you tighten the axle nut and the wheel slides axially? What is the nut tightening on, the end of the threads?
Definitely check the nut is the right way around. I don't recall whether reversing the nut makes it fail to go on far enough to clamp up, but you for sure won't have much fun mounting the wheel with it backwards.
The axle nut absolutely must tighten up hard against the spacer that goes into the oil seal on the bearing retainer.
The stack of parts must be clamped securely on the axle by that nut.
This goes:
axle bolt head - speedo drive centre - hub bearing inner race (no retainer) - hub spacer (inside hub) - hub bearing inner race (with retainer ring) - spacer (in seal) - axle nut.