If it is set at 50 and you have it set to SSB mode, then it should be putting out full power, especially if it is putting out full power in FM mode. Not sure what could make it put out 50 watts on FM but 0 on SSB on the same band, unless the SSB mic gain was turned down, but you said it is putting out full power on SSB on HF, correct?
73 John AF5CC