Welcome to the forum!. Hmmmmm.....I'd be really interested in this seeing a pic of the slug eater....aka "Suzy". I haven't heard of a garter that took slugs, but not earthworms?
One thing you might try (haven't done it yet myself, but don't have any slug eaters)...ZooMed sells canned snails that you could try out. I've fed them to my Northern Bluetongue skink before and wondered if they could be used for slug eaters like Northern Browns (Storeria dekayii) and such. Might be worth a try.