The command is not case-sensitive and hence may be typed in uppercase or lowercase.
How are you loading the program? (AppLoad / Startup Suite / ACADDOC.lsp / ACAD.lsp / MNL)
Does the function definition (defun) have a 'c:' prefix?
What program are you attempting to run (please provide a link to the program rather than post it here), and what command are you using?