It is only the underground tours of the Colosseum that sometimes get cancelled. My experience was a year ago, but if the method for tour companies to get tickets hasn't changed (and I don't believe it has) then I think there is still at least a 50-50 chance the tour will get cancelled or modified just a few days prior to the scheduled date.
They have the same problems getting tickets that you had. Their past practice has been to sell the tour, even if they don't have tickets and hope to somehow get some. If they are lucky, you go on the tour. If they are not, then there is the cancellation.
My outcome was actually fine. I realized that my regular tour of the Colosseum was quite satisfying and I really didn't feel as though I had "missed out" by not going to the Underground.