Carol, that's interesting information. On a whim, I did some looking around....
I see that WOI hasn't included any reviews of this tour on their website since January, and they've blocked all the dates for their underground night tour as well. Dark Rome is showing no open dates for their tour which includes the underground.
Context's private Colosseum with optional underground tour contains this text:
Tickets for the Colosseum Underground are among the most difficult to
procure in all of Rome. They are released for sale by the venue one
week prior to entry date and sell out quickly. The Underground upgrade
is not guaranteed until we confirm ticket availability.
Through Eternity's daytime tours with underground contain this text in their descriptions...
Tickets are only released 30 days prior to the tour date. If we are
unable to secure tickets to the dungeons (underground), the tour will
run with entrance to the arena floor included (as well as the Forum
and Palatine) but not the dungeons.
....and they are showing no availability at all for their group tour through June. No night tour availability showing until July, limited to 2 nights a week, and this text: "Please note that the availability of timed entrances to the Colosseum's underground area are extremely limited. As a result, the exact time at which we we will enter the underground varies on each date."
Roman Guy appears to still be offering their daytime and evening underground tour but might be a good idea to keep an eye on the site? That independent tour companies have as much trouble landing tickets for this section of the Colosseum is something I've read before but I wonder if this latest availability issue might have something to do with an possible upcoming installation of the retractable arena floor, plans for which were approved last year, and scheduled to be completed sometime in 2023?
https://www.dezeen.com/2021/05/11/colosseum-arena-retractable-floor-rome/
https://www.npr.org/2021/05/03/993184621/roman-colosseums-new-floor-will-give-visitors-a-gladiators-point-of-view