Thanks for sharing the model.
Looks like the main issue is caused by the source term inside the Heave Wave Force Signal. It was set as a "repeating sequence stair", so it stays as a constant for a duration of 5*T you set, and then move on to the next value of the function. For the 60 sec you simulated, the output from this source remains constant until the last step, when 5*T=60. If you run the model for longer, you will see the multiple steps, each with the value of one element in the array produced by the HeaveFK function. I don't think this is what you want it to do.
Using the "Repeating Sequence" block instead. See the attached example. And the position is now oscillating.