The Denver Broncos have a red zone mystery on their hands.

Trevor Siemian is playing his best football inside the opponents’ 20-yard line. Siemian is 14-for-20 (70 percent) with a passer rating of 116.3 inside the 20, and 8-for-11 (72.7 percent) with a rating of 114.8 inside the 10.

Despite this, the Broncos are 8-for-16 in the red zone; that 50 percent conversion rate ranks them 21st in the league.

So, what’s causing the team to stall once they are inside the 20?

Offensive coordinator Mike McCoy thinks it might be the penalties that are hurting the team the most.

“We had two penalties down there. You’re third and manageable, and all of a sudden, you become third and long,” McCoy said. Overall, the Broncos have sustained penalties on three drives in the red zone. Of these three, the Broncos could only manage a field goal afterwards. “We all know when you look at that, if we score touchdowns these games won’t be close. That’s something that you look at [in] the past two weeks that we’ve played… and the number of points we’ve scored — we’ve settled for field goals, which we don’t want to do.”

The Broncos have shown improvement over the 2016 season. In 2016, the team’s red-zone percentage was 46.8 percent, good only for a 28th place ranking. The team looked like it was off to a hot start, going 7-for-8 in the first two games this season, and then Broncos fell off the track, going 1-for-8 in their next two games.

Siemian, however, doesn’t believe that the struggle is a reflection of what the team is capable of doing inside of the opponent’s 20.

“It was such a small sample size. It’s easy to talk about it a lot, but who knows? I don’t think that’s where we’ll be by the end of the year,” Siemian said. “The first two weeks, I thought we were really good in the red zone, then struggled the last two. I think we’re in a good spot.”

Siemian’s performance in the red zone has been fantastic, but the running game has been slowed significantly when in front of the opponent’s 20. On 26 runs in the red zone this season, the running backs have only mustered 65 yards, an average of 2.5 yards per carry. Inside the 10-yard line, the running backs are only averaging 1.8 yards per carry. This is in comparison to the team’s overall running game, which is averaging 4.4 yards per attempt. The Broncos certainly need to find some way to increase the running game’s output inside of the red zone.

The Broncos used this week and the bye week as an opportunity to review mistakes, hopefully finding the errors that are causing them hardship in the red zone, and correcting them moving forward.

“That was the great thing with the bye week. [We] look at those things and evaluate them,” McCoy said. “We talk about that stuff every week. We just have to eliminate the pre-snap penalties, and I can help them with certain things. We’ll do better.”

The Broncos look to improve their red-zone efficiency against the New York Giants on Sunday Night Football, Oct. 15 at 6:30 p.m.