Theres a tab under the seat that has to slide forward under a 'ledge' at the back of the tank when installing. Easy to miss, and you have to take the seat bolts completely out and try again.
If that's not the issue, clean the suction cup and corresponding area of the tank with windex or similar, and make sure there's no obstruction under the front of the seat like a wire or errant shoe or something. Good luck!
