Yep,
Check whether you've allowed to create child nodes of your document type for the content node you've selected.
Bit more in detail: If you have two document types ('Post' and 'Reply'), and wishes to create 'Reply' subnodes under a 'Post' node, than you should check (as in checkbox) the 'Reply' document type as a 'Allowed child nodetype' (Can be found on the 'Structure' tab of your document type).
Have more questions? Just ask!
Regards,
/Dirk
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