This^. More often than not there are two reasons a mate or master doesn’t want to help someone with assessments. One is when they come up at, and after being in the wheelhouse for 5 seconds ask to do an assessment while still completely oblivious to what is going on around the vessel. If you can’t figure out it isn’t a good time to ask, that’s the first sign that I don’t want to be sleeping while you are running the boat any time soon.
Second is the idea behind the assessments are that when you bring your book with you to get signed off you already know how to so the assessment with no prompting or outside assistance. The idea isn’t that someone holds your hand through it and you get signed off. You need to be able to do it on your own.
Honestly I see the above two issues far far more often than a captain just being a malevolent prick.