Hi Giulia,
We’ve had numerous threads about charging issues already. The short answer would probably be “It’s the cable, stupid”.
But seriously: You should use a charger which officially supports Qualcomm’s Quick Charge 3 together with a high quality, IF certified cable. I ordered mine from Fairphone itself right with my FP3 (because I think Fairphone deserved the accessory margin 
and both work like a charm.
Good luck, Thomas