Why my VCO amplitude not constant?

When I decrease the tail current, the amplitude becomes constant. Very bewildering, what I used is a common negative gm LC oscillator. Only NMOS transistors are used.
Thanks!

Any oscillator need some time to stabilize its amplitude. Some need more, some need less time, depending by multiple factors, but mainly by the bias current (open-loop gain) and by the feedback.
If you go further in time with your simulation you will see that at some point should be stabilized.