Or maybe the captain who refused your request and directed you to a line trainer did so because he did not feel like do extra than what he was paid for on the day.
He may have accepted another day,in any case is he obliged to answer your request.
One day you will be on the left with a greener than green asking you to fly manually when you just had a loooong week and cant be bothered,,,you will send him to the trainers too.
This is the nature of the beast we are dealing with and the attitude needs to change.
Both need to be willing to do extra work,YOUR attitude needs to be changed,the captain does NOT OWE you anything,join the ranks,and stop whining.