This issue happened to mine once before as well as only one wheel engaging, I bled the calipers and had no further issues. It's been years since worked with this but I believe there were two bleed screws and I opened the upside-down one on the bottom, I remember that because it didn't make any sense to me.
Anyways I figured this all out after needlessly replacing a caliper that ended with the same results as before. Looked into it and it was too simple
