APEX

I have an interactive grid, based on a apex collection. Now the end user should be able to change the title which is in the column c001.

So far I have the following code in the "save interactive grid data" process:

declare
collection_name varchar2(255);
seq_id number;
new_title varchar2(4000);
begin
    collection_name := :COLLECTION_NAME;
    seq_id := :SEQ_ID;
    new_title := :C001;
    case :APEX$ROW_STATUS
    when 'U' then
    APEX_COLLECTION.UPDATE_MEMBER_ATTRIBUTE (
        p_collection_name => collection_name,
        p_seq => seq_id,
        p_attr_number => 1,
        p_attr_value => new_title);
   end case;
end;

But I think that's not the problem. On double click on that column in the interactive grid is no changing possible. The Interactive grid just turns grey like the background of the page. Even though edit on update is allowed, no conditions on read only and query only = no.

Am I missing something? Thanks in advance.