In certain summoning events, you can only summon a Focus Hero of your choice after summoning 40 times or more during a summoning event and ending your summoning session if you are subscribed to Feh Pass.
More details are included in the notifications for each summoning event in the app and in the Feh Pass features introduction page (linked below).
https://fehpass.fire-emblem-heroes.com/en-US/system/#no1
You will also not be able to summon a Focus Hero of your choice once the summoning event in question ends.
If you could have summoned a Focus Hero of your choice but didn't before the summoning event ended, please contact us by tapping Support at the bottom of the page and using the Inquiries form and we will investigate the issue.
Please include:
・ The name of the summoning event in which you were unable to summon a Focus Hero of your choice. (If you do not know the summoning event name, please include the duration in which the event was held, who the Focus Heroes were, or other identifying information.)