Q Addict, apologies if you aren't using the CG w/SFB. Can't remember for sure if that's what you're using.
If it the CG w/SFB, open the exhaust vent wide open. Open the SFB vent wide open. Now the temp in the main chamber should rise. Watch it carefully, as the temp approaches 220 start closing the SFB vent. You'll have to play with it to get the right amount of opening you'll need to sustain the heat. Just depends on the weather conditions, for one thing.
If you still can't get the temp to rise, try holding a fan or hair dryer at the SFB opening. Getting more air in there will speed up the burning process.
Your charcoal basket should have worked. Now we get into questions and maybe's. Did you remove the knockout in the end of the CG before you installed the SFB? Maybe your charcoal has gotten damp. There's been an awful lot of high humidity over the last few days and heavy rain in some parts of the country.
Did you let the butt warm up a bit before you put it on? If it was straight from the refrigerator, a lot of your heat is going towards heating that up. Your temp was a little low when you started, so that cold meat brought it down, I'm speculating.
Hope there's something in this is useful. And, that by the time you read this you'll have already figured out the problem. Do keep checking in.