Duplicants for some reason will refuse to repair certain pipe, regardless of what priority you have it set to. For example, here they can be seen repairing damaged (but not broken) obsidian insulated pipe elsewhere (which they have been doing for several cycles!) while the broken pipe at '!!' priority remains untouched, even though several dupes go idle:
Build pipe (not sure if it need be insulated or not), and break some of it by boiling a fluid in it. Watch as duplicants refuse to repair it.