I've had this happen to me often. For some reason or other the skimmer just decides to stop skimming. I think it knows when I think it's helpful and when it's getting in the way so when it knows that I'm finding it helpful it just quits to maintain it's annoyance.
Two things to try to kick the Skimmer in the pants:
1. Simply go to your project icon in the Browser and double-click it. This will reload the project (sometimes so fast you can't tell anything happened) and the skimmer will start working again.
2. You left Transform on. I have a habit of hitting Shift + T for the transform mode of a clip and forget to hit the Done button (or the blue Square icon button in the Inspector in the Transform section) to turn Transform off. While Transform is on, you don't seem to be able to Skim for some reason.
3. The third thing to try of the two things to try is to tap the "s" key in case you accidentally just toggled Skimming off while you try to reconcile some of the legacy Keyboard Commands with the new (and sometimes not only non-sensical but completely opposite) Key Commands.