Is anyone having trouble getting the "Substitution" function to work?

Hi everyone,

I'm working on setting up our Diploma (actually, 3 of them), but I am running into issues getting the Substitution function to work. I created a course called “Grade 9 English” to use for transfer students so that they show a credit for a course taken at a previous school when looking at their diploma requirements. Only problem is … there seems to be no rhyme or reason to which courses are/are not available in the Substitution area. Is there a setting or something that I'm missing? What is the general Best Practice on using the diploma requirement feature?