Android Thunderbird app not synchronising all emails

I have numerous Ionos email addresses. No problem seeing all received emails in Thunderbird app on my Windows PC. But Android app does not show some emails, despite synchronising numerous times. Problem is often with emails not showing in the unified inbox on the Android app. Also a problem with sent emails not showing. Today I looked for an email I had sent four days ago, and it was not in my Android sent folder. Synchronised, still not there. Synchronised again, still not there. Went online to Ionos and opened the sent email. Synchronised Android Thunderbird again and then it appeared. Why can't Anroid Thunderbird see certain emails? Problem seems to be especially bad with emails I have previously opened in Thunderbird on my Windows PC. Doing so seems to make the emails invisible to Android Thunderbird.