Something sounds wrong, the pad should have steel mesh, and a suitable footer, and 3000PSI or better concrete. Our spa has about 500 gallons of water (at 8 pounds per gallon?) this is around 4000 pounds, and with the original weight of around 1000 pounds results in a flooring load of around 1 to 2 PSI. If the pad is failing under these conditions, I would be contacting a lawyer to sue the original contractor. They probably did NOT correctly compact the soil, and used substandard construction.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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