It sounds like either your GUIDs need refreshed or your Base DN accesses a tree of AD where these users that cannot access the system are not in.
Refreshing GUID: Refreshing Presentation Catalog GUIDs | OBIEE 11g
Base DN: Check to see if your base DN references any sub-trees of your AD structure. I'm not sure how big your AD environment is, but if you have fewer than 5000 users, try to expand the Base DN to the entire domain by setting it to: Dc=domain_name,dc=com
There are other things that can cause this error, like a rename of an active directory user or some other item where a GUID refresh won't work. Here you either need to rename or forget the user using the catman command-line tool.
you can also check the login user adam vs ADAM(username in small vs capital letters).. most of the times this was the issue with AD users not accessing the OBIEE
try to paste the saw.log or check it from your end along with Adminserver.log which will help in resolving the issue. Make sure you have all the settings right
+1 on Sunil's suggestion. We used to have problems with caps user names all of the time.
We have resolved this issue by setting the group search to onelevel. thanks for all your help.