switch acces point via Iphone

Hi 

I have 7 acces point sophos and when my users changed place , the iphone keep the old AP and by consequent the wifi is slow and stop few minutes laters.

they have to switch off / switch on WIFI in order to connect to the near AP.

Whereas with my Huawei phone, there is no probleme ! it switched to the new AP in few second..

All AP and controler are up to date.

Any ideas please ?

thank you