Why does instagram not let me follow certain people?

[{Denise.'}]

When I try to follow certain private instagrams it sometimes says requested then goes back to saying follow...

[R]

Maybe it means that it has to be approved by the person you tried to follow before you can actually follow them, but I don't think I know. Best of luck!

[Marcos Hernandez]

This could be due to a loading issue or the fact that they immediately denied, but the second option is rather unlikely.

Try:
a) refreshing and going back to that profile.
b) restarting the app and trying again.
If neither of those work, it's a loading issue with the app. Report it by contacting them via email, page, or by posting a review explaining the issue in the app store.

Good luck.