How do I achieve proper threading on Thunderbird?

I'm running Thunderbird 24 on Ubuntu 12.04 64 bits. I have installed the Conversations extension, in order to group and visualize conversations based on threads, in a similar way to what gmail does.

It works fine, but it seems to depend on Thunderbird's internal native thread grouping mechanism, that is itself a bit flawed. In the example below, this thread include four different subjects.

I know many will say the threads are grouped based on an internal flag and if a contact of mine lazily uses a previous message, click reply to obtain the address list of an existing message and use it as a basis to compose a new message, with a different subject, then the new message will be automatically threaded as a child of the original message.

The problem is:

1. I can't educate ALL my contacts. Simply impossible;
2. Thunderbird is grouping messages with different subject lines
3. I have received automatic messages from external services, (meaning it's impossible they had hit "reply" on a previous conversation) and even then it gets grouped on a wrong thread.

Is there a way for me to improve the way Thunderbird will group threads? Or alternativaly a way that I can manually exclude and include messages from threads (that doesn't involve having to manually edit headers)?

It seems to happen frequently when I move threaded emails from a folder to another.

Solved by a simple two-step operation :

1. Compact the folder this thread is in (right-click then compact).
2. Repair this folder (right-click then properties).

You should try to repair first, but for some reason it may fail. Then just compact & repair.