The stolen video thing is actually a clue. It's not your content that's the problem, it's that IG doesn't know your account yet.
When someone with an established account reposts your video, IG already has data on them - engagement history, how long people watch, etc. So it takes a chance. With a new/small account it just... doesn't bother. Seen this happen a lot with cross-platform creators.
Couple things that actually moved the needle from what I've seen: be more active in Stories (even dumb simple ones), leave real comments on bigger accounts in your niche - not "great post!" but actual thoughts. IG notices when people engage back with you.
And try tweaking the hook for IG specifically. TikTok people give you 3-4 seconds, IG is more like 1-2 before they swipe. Same video can flop just because the first frame doesn't stop the scroll)