Manchester City broke the Football Association’s anti-doping rules three times in less than five months, the FA has said.
The Premier League club also failed to take notice of a written warning sent three weeks before the final breach.
City were fined £35,000 last month but the written reasons have now been released.
A first-team player missed a test on 1 September 2016 because the hotel address provided was no longer correct.
In addition, City also failed to inform the FA of an extra first-team training session on 12 July 2016, while anti-doping officials were unable to test reserve players on 7 December, 2016 because six of them had been given the day off without the FA being informed.