It was a cover-up by the Revolutionary Guard:

The first picture comes from the Iranian news website Jamejam, and clearly shows one missile has failed to take off. It sits forlornly on its launcher while its three companions power into the sky. But it was the second image that was released by the PR arm of the Revolutionary Guard – and it seems to have been digitally enhanced to wipe away all traces of the embarrassing dud. Gone is the faulty rocket on its launcher, to be replaced with a fourth successful launch, complete with billowing clouds of exhaust and desert dust.