When I installed mine, I had a very similar issue.
Mine was sitting to be sited on a block paved patio, which was laid over 15 years ago by the previous home owner.
Needless to say, it was no where near level, with quite a pronounced hump in the middle.
This resulted in the tub not touching the patio around it's whole outer circumference.
We ripped up the patio and relaid it.
Of course being block paved, each block has it's own profile.
Mines been down about 2 or 3 months without any issues.