'Open in conversation' working one day and not the next

Hi everyone, I use the 'Open in Conversation' feature a lot in Thunderbird but it has suddenly stopped working for me. Emails on 20th November and before are showing it as an option (either by right clicking email or through drop down menu) but emails on 21st November and since do not give me the option (not present when right clicking and greyed out in the drop down menu). The emails I have tried since 21st do have conversations and you can see the email trail if you scroll down the email. If I try Ctrl Shift O, then that does not work for post 21st November emails either. Does anybody have any idea why this is happening, and why I can access the feature for some emails before a certain date but not for those past that date? Thank you very much, Matt