When I decided to try running in the 300 range on my wsm I could never get it that high and maintain using kbb. Even with everything wide open and no water in the pan. Once moving to lump I had no problem getting there. I'd try switching fuel and see how it goes.
WSM 22 ~ WSM 18 ~ Maverick ET-732 ~ Flaming Fast Thermapen ~ Weber SS Performer ~ Weber 22 OTS ~ Weber 22 OTS ~ Weber 22 OTPlus ~ Weber 22 OTS ~ Weber Q300
When all else fails just ask yourself, WWGALD???