High engagement rate but low follow back

Recently, i found some of my accounts, they follow 400 every day, but the follow back rate is to low.
I let them follow the followers of some big accounts. The big accounts's engagement rate is selected no less then 2%.

So i checked the date, and found,
sometimes, i follow 100 big account's follower, only 2 or 3 follow back. the big account's engagement rate is 4%.
Sometimes, i follow 30~50, and 0 follow back.
It seems there is no connect between engagement rate and follow back rate.

I even found that, one account, it has 27k followers, but one of his post gets 45k likes, some of the post get 10k likes, the average engagement rate is 33%, but the follow back rate is only 3% when i try to follow his followers.

Don’t target that high followers accounts,most of them are filled with bots

-try to follow accouts im your niche with 10 000 -100 000 followers
-adjust the follow filters,accounts followers/followings its best to stay under 1000 followings
example 50-1000
-Content