Week Calendar uses iOS' native synchronization functionality, handled and managed by Apple, so this is a general iOS problem.
Calendars Hidden
Please ensure (all) calendars are visible.
- In Week Calendar, tap on the Calendar icon
- Ensure each calendar has a check mark next to it
Data still available? (99% of cases)
To verify if your events still are available you can check PC, Mac or online.
http://icloud.com/calendar
https://portal.office.com
http://www.google.com/calendar
http://www.outlook.com
https://www.yahoo.com/calendar
Reset Sync
- Go to device Settings
- Go to Calendar (or Mail, Contacts, Calendars in iOS9 and below)
- In the Accounts section, select the account which is not synchronized (iCloud, Exchange, ....)
- Reset / Re-sync the calendar data by toggling the account OFF and ON again
- Now reboot your iOS device afterwards, forcing it to sync.
Push & Fetch Settings
If your calendar still isn't updating please check your update settings, see https://weekcalendar.zendesk.com/entries/30967322 for details
If all above fails
Instead of toggling calendar Off & On again, please Delete the account.
Then reboot and Add the account again.
iOS8 Wifi Issue
- Delete the account.
Settings > Mail, Contacts, Calendars > iCloud, Exchange, .... account > Delete Account - Reset Network Settings
Settings > General > Reset > Reset Network Settings
The phone will reboot. - After the phone reboots
In case of iPhone, don't add any WiFi networks, until after you set up your account again
Set up your iCloud, Exchange, .... account again
Settings > Mail, Contacts, Calendars > Add Account
Fill in all your settings
After that, your account should sync up. - Once you've verified that the account is working, then you can add your WiFi network back in, and it should keep working.
Source http://www.imore.com/itunes-wifi-sync-not-working-ios-8-heres-how-fix-it
Local Calendar
If a local calendar is invisible this most often can be solved by rebooting your device.
If this doesn't solve your issue, the only way is to contact Apple since Week Calendar is using Apple's native calendar database.
Yahoo Calendar
If you are using a Yahoo calendar, please see https://weekcalendar.zendesk.com/entries/23274166 too.