Do you get any mail from the at job? Any output (stdout or stderr) produced by the program will be mailed to you when the process exits. So, if you are not getting any mail, perhaps your job is stilling running and the ps command is just not locating your process. Try using:

pstree -p
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and look for children of atd. You can also use the atq command to see if your job is still in the batch queue.