Why do some of these station's IBOC go off for a few days and then come back on?
NO one knows! ( Officially )
Possible and quite plausible reasons:
1. Encoding has failed and all hissing is in vain. It can actually hiss uselessly for days with no decode possible.
KFUO ? the Lutheran college in St Louis had this and *sparkles* beyond the hiss to about 60 khz each side. Reboot did not help....they turned it off for quite a while.
2. Encoder hardware has failed and waiting on repair/replacement/upgrades.
3. Antenna system bandwidth/spectral regrowth issues, and being worked on as part of upgrades to AM HD, each step reducing
to some degree the intrusiveness of the hiss as perceived to local listeners (step 1) and less splat on 2nd adjacents,
4 No one noticed it was off. ( It is sad but some radios are so crippled by design that you can't hear anything above 3 khz. )
5.Live sports broadcast, "forgot" to turn back on/wouldn't boot.