Read/write sync writes PC to smartphone, not the reverse

[renl60]

I've set up BirdieSync 3.0 beta 24,
to sync Thunderbird v38.2 on my PC with my calendar on my Galaxy S5 smartphone running Android 5.

I have both the calendars on my PC and on my smartphone set to Read/Write.

But BirdieSync only writes events from Thunderbird to my smartphone. It fails to write events from my smartphone to Thunderbird.

Any idea why?