There are some common causes of issues due to the nature of Geo-services which can cause unexpected delays or behavior.
A troubleshooting guide for Geo-services can be found below. In addition, if you are using voice service at the time, some cell providers block data usage during voice calls.
Geo-services is enabled on a per device basis. On the other individual’s phone, enable Geo-services and then you can select that phone for notifications/reminders