Although virtual school was pretty successful it was not a real virtual school but a data bank of material because as Graham Davies pointed out it was lacking a good tutor support system and high quality learning materials (at some departments).
I agree with John Simkin that the failure to persuade more countries to get involved in the project at the beginning was a major handicap. Countries got involved at different times some of them not having clear ideas about it, and the way they should recruit and support teachers. I have heard complaints by teachers that they were not supported at all by their ministries, they didn’t have any release time from teaching duties and it was extra work for them, they had difficulties acquiring money to attend / organize meetings, some of them had even difficulties to get leave from school to attend the meetings!
And of course a major drawback was that the people who represented the ministries who were bureaucrats were lacking classroom and ICT experience and whose decisions many times didn’t have an educational but a political orientation.