high power amplifiers are their own special beast. the picture does not tell us much, other than there are 4 high power FET transistors in parallel generating the 600 watts.
at this power level, something might have burned out. A directional coupler at these frequencies would be some sort of lumped element thing with a ferrite and windings on it. a capacitor might have microcracked, and works fine until it heats up, then becomes an open circuit. a solder joint might have let loose. a detector diode might have blown up, and so on.
With the RF power off, there might be a way to inject a smaller signal back into the output port, and see if the reverse power detector is actually detecting power. Somewhere there is a circuit in there that converts reverse power into voltage, and you could measure that circuit.
you will have to contact the manufacturer for an operating manual, that hopefully has a schematic diagram in it. get that, and tell us what the reverse power detector portion looks like.
btw, there is probably a pretty high drain voltage on those output fets--no touch!!!