Assuming there is a threaded end on the cable that mates with the end of the valve pull rod, it is possible for it to come completely undone so pulling the other end of the cable won't move the valve.
May work if you push it in and rotate clockwise a few turns.
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RVs. USA - Airstream Cutter; in Australia - MC8 40' DIY Coach conversion & OKA 4x4 MH; in Germany - Hobby Class C; in S America - F350 with 2500 10.6 Bigfoot camper