Cams stuck in Ikea case
If the index mark is aligned with the stud when the panels are assembled, and the cam is turned clockwise, it should pull the panels together. The photo shows an unusual gap, which may exceed the pull range of the cam. It also appears that the cam has been turned counter-clockwise based on the position of the index mark. Alternatively, the cam has been turned almost 360 degrees clockwise, which is excessive in this application.

The split in the bottom panel is an indication that the cam is applying leverage to the engaging stud, rather than connecting to the collar of the stud. If you are unable to force the panels closer together, consider to back a few turns out on the stud to allow the collar to engage the cam. This will pull them together more effectively.