There's a valve on the first stage that they're concerned about (looking at a "signature" problem, whatever that means). They think they can resolve their concerns and still make the launch window (which is a little over an hour long).

Clock is running again now. Delay of roughly 20 mins to 22:55 UTC launch.

Edit: so they counted down to T-6min then stopped and reset again to T-13mins due to a first stage internal power problem. They're waiting on trajectory coordination before trying a third time today.

Edit 2: and they've aborted again at T-3:40 due to what looks like a LOX problem. That'll be it for today; the launch window has elapsed.

For info, even in a completely "nominal" launch there are holds. This is particularly true when you have a tight launch window. The idea is to build some slack into the countdown to allow for the possibility if correcting any unforeseen problems. This is possible because at certain points in the countdown you can safely wait for fairly long durations. So the countdown is started early with respect to its launch window, and if any minor problems crop up they can be corrected during the planned pauses without slipping the launch time, and hence missing the launch window.