One simple thing to check for also that I had to run into in my last
house with a hot water system: Check the return lines where they join
back up with the main return "trunk". Somewhere near there may be a
balancing valve. Probably looks like a ball valve but might not have a
handle and would be turned by a large flat screwdriver. If someone's
been "adjusting" these, they could very well have closed off some of
the branch loops. There may or may not be these valves here or on the
supply end of the individual loops. Just something quick to check for
before you start tearing your hair out.