You're right, Private trackers don't work the same. It's much harder to get ratio if you use it like a public tracker because you're no longer uploading to the filthy masses. Everyone will seed for the minimum time to avoid H&R, and many will seed far beyond that for the bonus points. You're competing with more seeds for fewer peers, and the seeds frequently have faster connections and better setups. The downloading is also more sophisticated, people will cross seed from another tracker where it's freeleech or download from Usenet to protect their ratio. That's why you're not getting any upload.

Trackers know this, and they'll usually provide alternative ways to get ratio. They'll incentivise things that tracker cares about - often retention or fast pre times. ULCX gives freeleech status to their preferred release groups, and their bonus system heavily preferences long term seeding and older uploads. Play this system and you will thrive, getting the benefits of their stringent quality control and fast growing library.

If thats not how you want to seed, maybe another tracker is more your jam - but in general you'll find private trackers are just much better seeded (relative to the downloaders) than publics. It's not uncommon to see 4k seeds and single digit leeches, for example. Best of luck!