Open Slow Time Club. It checks your schedule and puts up any block that should be running right now.
If it keeps happening
Open the Focus Session in Slow Time Club and check that today is one of its days.
Check your plan in Profile. If the free week ends without a subscription, blocks pause until you subscribe. See What happens when my free week ends?
Restart your iPhone. This often clears a stuck background task.
Update iOS: Settings → General → Software Update.
Turn off Low Power Mode during sessions if you can: Settings → Battery → Low Power Mode.
Keep your clock automatic: Settings → General → Date & Time → Set Automatically.
If your phone was off at the start time, or you've changed time zone, see I changed time zone or my phone was off.
If none of this helps, go through the full checklist in Blocking isn't working: start here.
Why this happens
Sorry about this. When a session is due to start, iPhone is meant to wake a small part of Slow Time Club in the background so it can put the block up. iPhone decides exactly when that happens. Sometimes it's late, and now and then it skips it altogether. That's more likely when the phone is low on memory, on older iPhones, or just after an iOS update. Other screen-time apps run into the same thing, and Apple knows about it.
Opening Slow Time Club catches up straight away. Before an important session, it's worth opening it once at the start time to be sure.
Related: Blocking isn't working: start here · Focus Sessions: how do I set up a recurring schedule? · Blocks are unreliable in Low Power Mode, on older phones, or in games
Still stuck? Contact us from Profile → Help and Legal → Contact support and include your User ID (Help and Legal → User ID).