For birthdays originating from contacts
These birthdays are handled by iOS itself, Week Calendar only visualizes entries stored in this database.
If recently added birthdays are not visible in your calendar this can be solved easily.
- Close Week Calendar and other opened apps
- Reboot your iOS device
Now all your birthdays should be visible again
Birthdays without a year
Apple allows you to enter birthdays without specifying a year.
This results in all kind of errors varying from duplications or showing birthdays on the wrong date.
So, please always enter a year (even if you don't know the exact one).
You can ask Apple to make the year an obliged field by giving them feedback here: