Fragment not locked

I have a fragmented table in Mnesia. When I try to add a new fragment, as
described in documentation:

 

mnesia:change_table_frag(name, {add_frag, [node()]}).

 

I am getting an error

 

{aborted,{"add_frag: Fragment not locked",5}}  

 

What could be the cause of this? What can I do to add a new fragment?

 

Thanks,

Wojtek