There could be a number of reasons. Going from NG to LP is not always the easiest. Most of the later grills come with a continuously variable valve BUT unless you look at them you don't know for sure. If it has "stepped" or fixed orifice valves you cannot change/convert from NG to LP without complete manifold and valve assemblies. You could have also wrong sized orifices too. I remember the first time I converted a grill IIRC it was a 48k btu grill and stupid me I made the orifices for 48k PER BURNER (I think I was drinking at the time) it was a blow torch LOL. So new set of orifices and properly sized and all well.
Odds of your grill having fixed type valves is pretty slim BUT still possible. All depends on which Chinese co Weber sourced them from.
Here is a chart
http://andersonforrester.com/conversion-chart/ that will give you reference for the conversion.
You may also have the wrong type of regulator. You can buy high pressure regulators and low pressure types. You need a low pressure type 11"WC.
Good luck sounds like you're gonna need it