BBQ Guru DigiQ Dx2 Help

I tried this out yesterday and I could not get it to maintain a high enough temp 375 f.

I was doing a high heat brisket
- 15 lbs Stubs
- Minion method ( hole in the middle and started with about 50 briquettes)
- 22" Weber Smokey Mountain
- Water in the pan

- I needed to open the vents on the bottom to stay around 350
- The unit seemed to be cycling when I set it to 350
- I opened the door to get it to 400 to see if it could maintain the temp with the vents closed and it dropped like a rock
- The fan was running and pushed all the in
- I unplugged everything and plugged back in
- I tried turning off the open lid feature


Any thoughts?

I'm I expecting too much?
Tried calling the company yesterday and today about 6 times and my favorite "Voicemail"

Thanks for any help you guys can offer

So just pile the coals in and spread a full chimney over them?

Isn't 1 full bag of Stubbs 15 lbs enough? Or how much more should I use for a high temp cook?

And thanks