I had a problem like that before, users were able to get into their home shares but a share for specific users like Lab share in your case not. What I did it was to create in AD a group (secretaries in my case) whose members were those ladies I wanted to access that particular share, and on the samba box I did:
chmod -R root:secretaries /hom/shares/Lab
I also had to selinux enabled 0 and setenforce 0. After that those ladies were able to access their custom share.
regards,
Linux user number 433512
"Free as in Freedom":
http://counter.li.org
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