Juice from meat extinguishing charcoals when grilling?
As a fellow Webber'er and long time griller, I'm going to wager one of three things going on here:


Your coals aren't hot enough.  One of the biggest problems I see with folks and charcoal grills isn't waiting long enough for the coals.  There's should be no visible flame and slightly more than half of the coals should be white.  In a chimney starter - this is around 25 minutes for me.  If you're using another method, YMMV.
You don't have enough coals.  If you've just got enough coals to spread a thin layer across your grill, then they won't keep heat well and they'll cool down fast. If the liquid hits them too much here, they won't heat back up well. They should be grouped together somewhat and generally a couple layers thick for best heat.
There is waaaaay too much marinade still on your meat when it hits the grill.  If its been soaked properly in the marinade, you don't need to have it still be gushing fluid when it hits the grill.