Please make sure your data appears as you'd expect in the built-in Calendar app. If it doesn't, make sure your accounts are configured correctly in the Accounts & Passwords section of the Settings app.
If you made sure that your calendars and events are visible in the built-in Calendar app, also make sure you've allowed Calendar+ to access your events. You can verify this by going to the Privacy->Calendars section of the Settings app.
If your events still aren't showing in Calendar+, try turning your iPhone/iPad off and then back on. To do this, hold down on the power button and swipe to turn off the device when prompted.
To make sure you always see all events, do the following:
- Start the Settings app
- Choose Calendar -> Sync and then tap on All Events
- Choose Accounts & Passwords -> Fetch New Data
- Scroll down to the Fetch section and choose a schedule that suits you. For example, setting this to 15 minutes will tell iOS to refresh your calendars automatically every 15 minutes.