Help Needed Regarding Followers

Hi everyone.

I’m quite new to WordPress and was wondering whether someone could help me regarding my followers. On my phone I receive loads of notifications of people following my blog, an example can be seen here:

https://tinypic.com/r/6ftl4o/8

The problem I’m having is that it gives me the option to follow some people back however it won’t let me follow the others (as you can see). I know there is probably an obvious answer but being new to it, I have no idea why.

The people who are following me, is there a reason why I can’t follow them back? Why?

Thanks ??