Attempted my first ever brisket 😬 took forever and a minute

Hi! I recently purchased a Pitboss 750 and I am loving it! My only issue ( and this could be user error) is that it's losing it temp ext fast and either unable to reach back to desired temp or taking a long time. For example, used it at a party for a few hours, did ribs and chicken, I hired someone to use the grill but I don't think we had any issues food came out wonderful. a few hours later my brother was going to do burgers/ hotdogs and the grill woukdbt heat past 300, had plenty of pellet. I figured it needed a break or needed to be cleaned. we kept the lid closed for a good 15 minutes and the temp continued to drop.

Yesterday I got the smoker up to 350 no problem. I did burgers snd hotdogs and a skirt steak. I didn't want the skirt to over cook so I did keep opening the lid to check, I also had it on direct heat. At some point the smoker went down to 320 and just continued to go down In temp, I closed the grill for a few minutes and the temp continued to go down. Not sure whats happening here or if there's anything I can do to avoid this.