A thermocouple has the end that pokes into the flame, and a ‘tube’ running from the back of it. At the end of the tube is a fattened out bit with a captive nut. This end connects to the valve. If wiggling it makes it work then either the nut is loose or there’s a break within the tube bit. If it’s loose, tighten it. If the fault is internal, replace it
So in your diagram 908 screws into 912. If that nut is loose (unlikely) then tighten it. If the other end isn’t in the flame then move it. If wiggling it has any effect whatsoever then replace it.