First, disconnect the power leads from the load side of the breaker, turn it on. If it holds, turn off & reconnect.
Second, Disconnect power leads at hot tub, turn on breaker. If it holds, turn off breaker.
Reconnect wires, turn on breaker. If it won't hold, chances are the problem is within the hot tub.
Examine hot tub wiring for any obvious pinched wires or accidently grounded conductors. Is there a neutral connection to the tub? If so, make sure there is not an inadvertant ground anywhere after the GFI breaker.
If you still haven't found the problem, call for warranty service on the hot tub. The first two steps pretty much eliminated a bad breaker or a bad cable.
Few things are harder to put up with than the annoyance of a good example.