I have kegged a beer about a month ago. Primed it with 1/2 cup of dextrose. Just tapped it now and beer pours very foamy. When I pour it its nothing but foam but beer is flat. I am kinda new to kegging. Its my 10th keg and I have primed every keg this same way with 1/2 cup of dextrose. And it all worked just fine. This is the first keg that I got a problem with. Any ideas???