Turn rad valves off drain rad then open each valve and see if there is a flow of water from each one with water in system if not than will mean a blockage.
Steelkane are best i worked for dyno rod and they used them the thicker ones ane more solid but the thinner ones go round bends better dont bother with cheap ones as when on dyno rod had to get quiet a few out of drains that owners lost or broke.
Say i was doing a boiler swop.
10% b4 any goods ordered
say 50% when boiler on site / on wall not working.
the rest on completion.
been doing this way for 40 yrs no problem.