so I bought a weber 18.5 about 3 months ago and I'm new to smoking and I'm having trouble getting the temp to go above 225, it takes about 2 hours to get to 225 and then about 3 hours to reach 250 and after that I cant seem to get it to go any higher. is that normal or am I doing something wrong? I keep all the vents wide open but still doesn't seem to help.