Encrypting for multiple users

[Mohan Radhakrishnan]

Hi,
 
I am encrypting for multiple recipients.
 
gpg -r <user1> -r <user2> -r <user3>  -r <user4> --encrypt file.txt 
 
3 users are able to decrypt the files. The 4th user though is getting this messsage.
 
I am using GPG. All users use PGP to decrypt. Shouldn't make a difference ?
 
"It is not possible to decrypt this message because your keyring does not
 contain usable private key(s) corresponding to any of the above public key(s)."
 
 
It looks like the 4 th user is doing something wrong ?
 
Thanks,
Mohan
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