That's fairly normal. Try flicking the light switch off then on and see if it triggers it.
One of my ddm slim balast's would do that from time to time. I would get the initial flash on start up, but no light up. Usually I had to unplug it real quick and plug it back in. Almost ways worked. The bad part about ddm is there customer service. Your better off buying another ballast to replace then warranty the bad one and get it back when they feel they need to send it and have a spare.