I don't get Instagram

[Borkins]

How do you get followers?
I have a fitness business page and I've posted regularly etc
Some photos reach much more than others. Likes are random.
I don't understand how folk have thousands of followers!

Do you pay to sponsor posts? Do you utilise hashtags to attract the right audience?

I use hashtags

Don't pay to sponsor posts?

Ask people to share the posts with their friends? And get all your clients to follow you.

If it's fitness do you want local clients or to reach folks where-ever they are?

Follow people, they follow back, or at least look to see who you are. So, any major players/ people in your area who have fitness interests...
Comment on posts of others/ feature them in yours and they’ll share to a wider audience.

You need to work out the time you get most follows? Be very interactive with the comments you get left. Do lots of stories - like lots of other people who will then hopefully follow you - it’s hard! My daughter does an art style account (not to sell anything or as a business although she has had some lovely ‘freebies’ for doing #ad or #gifted). She had nearly 50k followers and is now down to 44k because she had stopped posting for a while due to illness and doesn’t always interact with comments left on the photos that she does post.

As a follower to lots of accounts I’ve noticed that if I don’t ‘interact’ by liking or commenting on a photo that person drops off my feed!! Bloody algorithms

Essentially you need to be producing high quality content that looks great and which people want and value. There isn't really a shortcut for that, and it can take time to build followers. Using IG stories is good but ideally doing stuff people are going to want to see. A few polls or interactive story posts, some videos on there too. Working out what people want and how to give them that is most of the work. If you focus on that, then followers will just sort of happen.
I'm assuming you're looking at other accounts and seeing what sort of aesthetic they have and how they are interacting with people that follow them? Can you learn anything from them?
Being selective and thoughtful with your hashtags is also good. Vary them up, explore new hashtags that will get eyeballs on your IG. Pick some with less footfall so you are more likely to be found, don't always use mega popular hashtags.
Good luck