Joanne,
According to the latest statistics over 7.4 million tourists visit the Colosseum each year. In simple math that equates to over 20k visitors a day without taking into account fluctuations of high tourist season. You can only imagine how may people are on that website on a daily basis trying to buy the least expensive tours available.
I'm sorry you're having so much trouble, according to the Coop Culture website they will be releasing more tickets in June.
MONTHLY OPENING
Every first Tuesday of the month :
June 4th at 3 pm (italian time): The pre-sale of tickets with scheduled entrance time for the ordinary access of the Colosseum, of tickets with scheduled entrance time to the Coliseum arena, in the monthly availability for July 2019 will be released both online and through the call center.
July 2nd at 3 pm (italian time): The pre-sale of tickets with scheduled entrance time for the ordinary access of the Colosseum, of tickets with scheduled entrance time to the Coliseum arena, in the monthly availability for August 2019 will be available both online and through the call center.
Every first Thursday of the month:
June 6th at 3 pm (italian time): The pre-sale for the underground, the belvedere and the combined underground belvedere tour, in the monthly availability for July will be released online only.
July 4th at 3 pm (italian time): The pre-sale for the underground, the belvedere and the combined underground belvedere tour, in the monthly availability for August will be released online only.
I hope you find this helpful!