They looked about the same size and shape in the pics. Angles I guess.
I've never known anyone that had to do anything special for the breeding of this species. Some people have had them breed with other species quite readily.
Maybe try a very large water change. That triggered my cons to breed last time (not that it takes much for them).
Feed them a lot of meaty stuff. Your temperature also may not be right to get them into breeding mode. I didn't see any mention of it but may have missed it.
I'd ditch the sea rocks as they'll probably raise your hardness, or do other things to your water quality. Maybe up the salinity or something. Ditch those, and get 4 or 5 flowerpots. Put them in on their sides, and viola! Convict love nest.