Hi Salwa,
This is due to “randomness” of the noise. In each separate simulation the noise generated is a little different. In each component that generates a noisy signal you can choose to keep the same seed for each simulation, so you will get the same result. In the Randomness tab of the Component Properties uncheck Generate Random Seed.
On the other hand it is generally more accurate to keep the box checked and run the simulation with a longer bit Sequence Length. This will keep the randomness of the signal but each simulation run should give much more similar results.
Regards