It could be a number of reasons. If they were friends you were looking to connect with, did you send them a message at all or just follow/like/comment?

How many accounts are you following? If it’s in the hundreds or over 1k?

Are they posting just personal stuff or is it art/business? If it’s the later, perhaps they only follow things related to what they are doing.

Also, are you sure they know who you are by your account name? A lot of people keep their account private but still accept any request sent.

It could be a number of things, but if you’re interested in catching up, send them a message! If they don’t reply and you’re not interested in a one side relationship of seeing what they’re up to, then unfollow