Well it's not a coincidence that his struggles started just two games before the conference season began with game 13. You kinda answered your own question without actually answering it...
As was suggested above - teams learned how to guard him. They also know if you take him away that we didn't have a lot of other weapons that could actually beat them. Make no mistake Jordan Barnes can shoot you into any ball game but he can shoot you out of any ball game.
The other part of this is Jordan Barnes / Lansing & Co. made himself / him easy to take away. Teams in conference play are generally more physical - many times they'd put a little taller defender on him, be physical with him or force him off the 3 point line and the game for Jordan Barnes was over before it started.
It's as simple as this - one of these two things has to happen. Either Jordan Barnes learns that he doesn't have to be a score first player to have an impact on the game. OR Greg Lansing needs to sit his ass down and let him watch the game. Barnes has far too much experience to be disinterested while handling the basketball and by forcing himself to take difficult shots. He's done a poor job of getting his teammates involved and his body language leaves a lot to be desired. Barnes didn't make more than 2 three pointers in a game in a single conference game all season and it wasn't for lack of trying. He just has to improve upon his shot selection and take fewer attempts. He can impact the game in other ways. If Barnes doesn't figure that out then Greg Lansing better figure out a way to help him figure it out...
If I saw Lansing right now I'd tell him as much - my guess is that he would stubbornly disagree with me and he will continue to let him shoot us in and out of basketball games most of the season. That's his right - he doesn't get paid to listen to fans. I just think you've got some better scoring options and Barnes can certainly be a better passer and facilitator than he's been in the past.