Most of the time the calendar was subscribed correctly and only needs a moment, or a setting in the subscriber's calendar app. Work through the case that matches what they see.
The calendar shows the feed address as its name
Google Calendar adds the calendar as soon as the link opens, then fetches the feed in the background. Until that fetch finishes, it shows the feed address as the name. It usually renames itself within a few seconds.
If it keeps the address as its name, ask the subscriber to remove the calendar in Google Calendar and subscribe again from your calendar page. Google tends to keep the first name it saw.
Google Calendar on Android shows no events
The Google button on your calendar page opens Google Calendar in the browser to add the calendar, which avoids a known problem in Google's Android app. After adding it:
Open the Google Calendar app.
Go to Settings and choose the subscribed calendar.
Make sure Sync is turned on.
Outlook does not open or shows an error
The Outlook (desktop) and Apple buttons open a page with a button to add the calendar. If Outlook does not open, or opens the wrong calendar, click add it manually on that page and follow the steps. If it still fails, contact support with the calendar link and the Outlook version the subscriber uses.
Yahoo Calendar
Yahoo Calendar cannot subscribe from a link, so the Yahoo button shows the steps instead:
Copy the calendar feed address shown on the page.
Go to calendar.yahoo.com.
Open the Actions menu and choose Follow Other Calendars.
Give the calendar a name and paste the address.
Click Save.
Events show on the computer but not on the phone
Some phone calendar apps only show the calendars you choose. Ask the subscriber to open the app's calendar list and turn on the subscribed calendar.
Events are still missing
Check there are upcoming events. Draft events are not sent to subscribers.
Allow for the refresh schedule. Calendar apps check subscribed calendars on their own schedule, so new events can take a while to appear. See Calendar subscriber update delay.
Check your subscriber limit. On the Free plan, calendars can have up to 100 subscribers in total. If you stay over the limit for 7 days after the warning, the calendar stops sending events to every subscriber until you upgrade or the count drops. See Free plan limits.
Check for duplicate subscriptions. See Manage calendar subscribers.
If none of these help, contact support with the calendar link, the calendar app and the device the subscriber uses.