Are you re-importing the same mesh after merging vertices or totally importing new one ?
Sometimes it can not read the new data or confuses between the old and new one, so import the new mesh totally instead of re-importing it.
Are you re-importing the same mesh after merging vertices or totally importing new one ?
Sometimes it can not read the new data or confuses between the old and new one, so import the new mesh totally instead of re-importing it.