Update wordpress in subdomain by ftp

Hello,

With normal installation of wordpress on the server there is no problem, the automatic update works properly. I add the the following:

ftp.mydomain.com
username: xxx
password: xxx

and everything work fine. BUT when I install the wordpress inside my subdomain, this thing does not working anymore, I mean the was to update directly by using ftp, why ?

Regards !
BB

I hope to get your help when you have free time, thanks!