It seems that the re-roll is broken, and stops at random points, preventing you to re-roll single enchants.

This sometimes happens after any number of re-rolls for a stat, after 2 or after 5 for instance.
This sometimes happens before re-roll anything, so that a specific stat can not be re-rolled from start.
There seems to be no apparent pattern, and any of the 4 re-roll-able enchants can be affected by this.