User appears in group's members- but not vice-versa

We are using XIR2 SP5. We have clustered servers using LDAP authentication.

A user belongs to a group. User shows up in the group in the CMC as well. BUT if you look at user´s profile within the CMC and click "Member of" to show user´s groups- the group does NOT show up. When the user uses Infoview and Webi they do not have the security rights that this group grants.

I read that it is good to reboot system but sometimes it doesn´t help. Do you have some root cause of that or more information to this problem or another solution?