As Runn uses a mass emailer system, your organization may have security settings may be blocking delivery of the emails (but it may allow the first few through and then block subsequent emails). If you suspect this is happening, please ensure your IT team add the Runn emails "invites@runn.io" and "noreply@runn.io" to your whitelist (Approved Senders list)
For smaller organizations- you may need this done on an individual basis by all users by creating an Approved Senders list or adjusting their SPAM filters.