Usually when that happens to me I didn't start out with enough lit coals. You might consider starting another chimney, taking your smoker apart and adding it once it has fully lit and ashed over. I would also add some additional unlit while it is apart i order to sustain the heat.
When I put a full chimney of unlit and a full chimney of lit in my 22.5 WSM, I top out between 325 and 350. Two chimneys of lit will get you 400 plus.
"Houston is a wretched little town composed of about twenty shops, and a hundred huts, dispersed here and there, among trunks of felled trees. It is infested with methodists and ants." -Emmanuel Domenech, Catholic Missionary, 1848