Thunderbird - reversing junk mail

I have specified criteria in the "message filters" to determine that specific *from* sender's names be classified as junk and immediately moved to the junk folder. This works great and saves the time and effort to move them manually.

However, for some reason, Thunderbird often takes a message that does not meet the filter requirements and assumes it is junk, then moving it to the junk folder. Nothing about the message should make the program think it is junk.

I have used the option *mark as NOT junk* on these messages but it continues to send these same messages to the junk folder anyway. Any idea how I can tell Thunderbird these senders are not to be treated as junk, and send to the inbox??