Active Directory user don't inherit permission from a security group

Hello,

I am not sure how to write this question.

I have a windows server 2008 R2 with Active directory.I share a folder to a group (ftpgroup) and my user user1 is a member of this group.When user1 try to connect to this folder he get refused.But if i directly add user1 on the NTFS permission of this folder it work.

I obviously miss something somewhere but i don't know where to look at.

Thanks in advance

Vincent