I found out what my issue was. Apparently programs like SpamAssassin and AMaViS are processes of Perl, not on their own. So the correct check command would be something like:
$USER1$/check_procs -c $ARG1$ -C $ARG2$ -a ‘$ARG3$’
Where the service check is: check_procs!1:idea:Perl5.8.6!/usr/local/bin/spamd
Using the 1: instead of 1 just indicates that I want a minimum of 1 process running with no maximum.