I understand that you want to make a simple Mass spring model. The model you attached, produces desired output, and I believe the problem lies in the selection of solver configuration. Probably you are using local solver and Backward Euler, as the solver type.
The Backward Euler tends to damp out oscillations, but is more stable, especially if you increase the time step. You can either uncheck ‘Use local solver’ or use Trapezoidal Rule Solver.
For more insight on making optimum solver selection you may refer to the link:
