You need to right click on your sub-assembly in the tree & select properties, then set the option "Solve As" to flexible.
RMB on the sub-assembly, and select "Component Properties".
Near the bottorm right of the pop-up you'll see a box marked "Solve as" with2 check boxes below it.
"Rigid" is ticked, change that to "Flexible", then click on "OK".
In 2014 you can right click your assembly in the tree and there is now a button in the toolbar that pops up to make it flexible.
Thank you for all your answers. All of you are right but I just can choose only one correct answers. Sorry about that