We all know Shannon CAN shoot, the issue is that he's shooting threes at a much greater volume than he was at Texas Tech. He was at 7.9 three point attempts per 100 possessions last season at Tech, and he's at 9.5 three point attempts per 100 possessions this season with Illinois.
So without watching every single attempt throughout his career, I'd reckon that in line with his overall greater usage as a "go to" scorer, he's taking a lot more three point attempts off the dribble and in creating his own shot, as opposed to shooting in a more catch and shoot fashion when he had a clean look. Variance always plays a bit of a role, but I reckon that is a much bigger factor in the decline in his three point efficiency.
He just really needs to cut those bad attempts (visible from the eye test). It's fine to have in the arsenal but it should not be his "go to". Since his sun run of a shooting stretch against Monmouth and UCLA (12-18 for three in those back to back games), he is shooting 25.8 % on three point attempts on a very high 4.9 attempts per game. That's an 18 game stretch spanning 10 weeks. He's really good at going downhill and either scoring or getting fouled, that's what he should be focused on doing when he touches the ball.
As for what Underwood was hoping/relying on coming into the season, probably wasn't counting on Goode going down with injury, probably wasn't counting on RJ Melendez going on a season-long slump (if you can even call it a "slump" at this point). Skyy Clark also had a good recruiting billing as far as being a dangerous jump shooter. Mayer was and is a pretty good shooter. Shannon was and is a three-level scorer (just see issues above), Hawkins was a guy who can at least keep defenses honest without necessarily being a true "stretch big" (he just seems determined to try and be one with his 7.4 three point attempts per 100 possessions). Epps was billed and is a guy that can help provide some needed shooting spark as well (and he is only a Freshman so that should get better).
Biggest issues from what we thought we had to what we do have are almost certainly that Melendez has performed way worse offensively than any of us would have expected based on what we've heard about him in practice and he showed in his limited sample last season, and that Shannon and Hawkins are not making good decisions as far as shot selection. These are hopefully fixable in that you hope Melendez can find some confidence and get his stroke together by March (I'm not going to hold my breath waiting for Goode to come back and be a savior after missing the whole season to this point) and that Shannon and Hawkins can play more within themselves. The only time I really feel like this team has a serious lack of shooting as far as keeping defenses honest is when 2 out of 3 of Dainja, Rodgers and Harris are out on the floor.