To launch the Ready for Check in PLM command in SmarTeam, you have to be set as "Responsible" on all the CAD Documents (Master and non-Master) attached to the Item.

Moreover, you must also take “Responsibility” for the CAD Documents attached to the Items that compose the BOM.

In other words, you must be “Responsible” of all the CAD Documents collected by the Ready for Check in PLM command (dependencies that are not already "Ready for Check in PLM" or "Released".)

Complete documentation: Ready for Check in PLM FAQ