If you slow down a clip by decreasing its speed in Properties you must first drag the clips to the right of it further right to make space for the extra time that clip will take to run. Otherwise it will overwrite them. When dragging make sure the “Ripple Trim and Drop” icon is selected, so they will all drag in tandem.
If you speed up a clip, the opposite will happen. Blank space will appear between that clip and the one on its right. This can be removed by right-clicking in the blank space and selecting “Remove”.