Iphone 6 16 GB flash not working properly

Hi,

I have somewhat strange situation.Bought second hand iphone 6, advertised as "flash not working". Fime ith me, it was quite cheap anyway. However, once while taking photo in front of church, it suddenly flashed. (God's hand???). Once only and never before or after. Now it flashes when someone calls at its own pace, but still doesn't work as a torch or as flash. So, it's obviously working, but for some specific purposes only.

To clarify the issue, flash works as a call note ( not all the time, though), but not as torch or flash while taking photos. Just to avoid some classic replies, I know how to use flash while taking photos, and how to use it as a torch... So, it is not an issue.

Any idea?