Almost all hot tubs use 230 volt AC power which comes from the power company in two power wires plus one common common wire. You get common 115 volts between a common and either other wire. "Half way" working could well be an attempt to run on only one of the wires. There is also likely to be a ground fault circuit breaker in the wiring which makes sure current going out to the tub is all returned via the wires and not through someone's legs to ground. If you're comfortable with an AC voltmeter check some voltages but understand that 230 volts is way into the killing range. Two people present and well insulated probes are important.
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