What Causes Missed Periods In 14 Year Old Inspite Of Taking Treatment?

Hi my daughter is 14 years now when her 11th birthday she got her period and it came regular for one and half year after that it stopped suddenly. Doctors checked her and tested all hormone levels and they did ultra sound too and not FSH and TSH tests. Every thing normal and she told it's quite common in young kids. She prescribed pill for 6 months. My daughter used for 5 months and then her period was regulated and she stopped 3 months back. Once she stopped her medicine her period didn't come till now. What i have to do now I am worried please advice me. Thank you