Drains

We are currently having issues with our dirty sink drains. They are functional however, we have a few that are draining at a snail's pace. The clog is not directly after the sink but where the wall boxes ties into the line after the sink drain. It appears that it is clogging somewhere there and then backing up into the sink trap. This building was built 12 years ago. Beyond us, the drain lines are cast iron. The only fluid that is being put down sink drains is bleach, vinegar and the occasional normal saline. Every month after disinfection I flush wall box drains with EZ Gel (RPC). Every wall box drain is draining freely, and we have never had a problem with drain flies. What we are finding is nothing like I have seen before. Usually, a clogged drain is nothing more than "chicken fat", caps and line paper and can be easily cleaned out. What we are seeing this time is hard almost rock like, yellowish debris. Whatever this is has been accumulating for a long time and has gathered in areas that are we can't access. We do have a contractor coming but they will not be able to start this project until early spring. Has anyone else experienced this? What the heck is it? How do we prevent this to happening again in 10 years?