We're happy to hear that you enjoyed Kadin and Richard's route. To unlock the Special CG gallery, you'll need to try out all choices in each of the boys' route. This includes the prologue as well. There are some choices that only appear in a particular choice (a choice within a choice), so you'll also need to go through those choices and variations. Once you've tried all choices and variations in the route, Richard's special CG would be unlocked.
If there are any other questions, feel free to let us know. And thank you for reporting that bug.