i wonder this about certain users in here

there's certain users who answer my posts from time to time, or the odd occasion, and their answers are useful and helpful and i tell them that and thank them usually, but they don't interact with me or reply to me that often, and i wonder why they don't answer my posts to help me more consistently or more regularly?

i realize that i can't 'force' people to, but there's times i really need help or advice with my post or question and they don't respond, and i know they're active on here at that time and wonder why they don't help me again like they did the last time they responded??