Unfortunately we do not get device-specific details as to what should be done by a participant to receive in-app notifications. If the notification does not work for a single individual but works for other people in the study it means the study settings are correct and the participant’s device settings have to change.
If the participant cannot figure out what changes are needed on her device, the alternative is to use Email or SMS notifications. They both need email or phone number of the participant to be recorded by Ethica and verified which might be against the Ethics application of the study. Also SMS notifications will incur costs both on the study (cost of Ethica sending those notifications) and on the participant (network charges).
Hope it helps,