I have exactley the same problem I currently have 4 Panasonic digiboxes all on old software, tried many times to force downloads to no avail.
As I am in the business of digibox repair I often repair digiboxes that when repaired find that software is an older version. On all other digibox makes they may take some time to upgrade to the latest version but do eventually.
Afraid Panasonic boxes after many attemps fail to upgrade.
As Sky operate the upgrade stream the answer to this is probably known by Sky. I do believe however that there is nothing wrong with these Panasonic boxes and the issue is purely software related(Sky operated).
Out of desperation I rang Sky technical and stated that I found an old Panasonic receiver in the garage and would like to use for Freesat but the software version was version 3.2.11. The person I spoke to said he would have to ask a supervisor, after a few minutes he stated that he had been informed that the only way that it would be upgraded was if I subscribed to a package of channels!
My own belief is that if you have a Panasonic digibox on old software it probably will be upgraded when sky do the next block software version download. So keep them dont bin them until that time.