My first instinct is to make sure you wired the breaker correctly, i.e., the coiled white wire attached to the GFI breaker gets landed on the neutral bus, while the grounded (white) that goes to the tub gets landed on the breaker itself. No intention to insult your intelligence here; it's just that i did this once myself when i wasn't paying attention.
Does the breaker only trip under load? If It trips whether or not the tub is running, disconnect the load from it and see if it holds. If not it's your breaker, if it does hold, without a load, next place to check is the tub.
You may want to call the manufacturer of the hot tub. I ran into the same issue one other time and if memory serves me right I had to use a GFCI disconnect provided by the manufacture. It had something to do with the way the neutral was tied in.