Can't get temperature past 300 degrees

What are you using to start the fire?   I have an XL and use a full Weber chimney to get it started.  For high temp cooks, I use a full chimney and for low/slow I only use maybe 1/4-1/3 get it red hot. For either H or L/S I dump it in the middle, cover with "old" lump then and add more to fill up the firebox.  

Also make sure you are keeping the stored lump dry with very little humidity. Humid/damp lump can cause this sort of thing as well.

do you use a chimney vent like shown below?



I've experienced that it took me forever(approx. 1-1.25 hrs) for me to reach 600ºF when my bottom vent was wide open along with my chimney vent "wide open" as shown below.


I concluded after a few start ups that it takes forever for my egg to heat  up and the simple solution was to just remove the top piece of the chimney cap(to get more airflow thru the egg) when starting your coals along of course with the bottom vent wide open. Ever since then my egg starts up to any desired temperatures in less than 30mins and then i would place the top chimney piece back on to regulate to my desired temp.

If you got the regular damper that came with your egg then I would remove the whole damper to start your coals.