Best answer
A broadcast skips a contact when sending to them would be refused or would break a rule, and the four reasons cover nearly every case: they are unsubscribed, the 24 hour window is closed, the channel is not usable, or they are not in the segment you think they are.
Unsubscribed. Anyone who sent STOP, cancel or unsubscribe is opted out and excluded. This is not overridable, and it should not be.
Outside the messaging window. On Instagram, Messenger and WhatsApp a free-form broadcast can only reach people who messaged you in the last 24 hours. For most lists that is a small fraction. On WhatsApp an approved template reaches the rest; on Instagram and Messenger there is no equivalent.
Channel not usable. If the channel is paused, blocked or needs reconnecting at send time, its contacts are skipped rather than failed one by one.
Segment membership. A segment is evaluated when the broadcast runs, not when you built it. A contact who lost the tag yesterday is not in today's send.
The delivery breakdown shows which of these applied, so start there rather than comparing counts. If the skipped number is large and the reason is the window, that is not a bug: it is the platform, and the fix is collecting an email or phone number for the audience you actually need to reach.