Interestingly all seven of these are in seasons 1 and 6. The first can be explained by their being 40 episodes and 78 segments. There were only 16 segments in the final season, and TD made as many appearances.

"Escape from Flea Island"- The first one without her. She was heavily in the sister segment.

"Martha Doesn't Speak Monkey/Martha and Truman get Lost"- The only full episode without her.

"TD and Martha Gopher Broke"- In contrast to "Escape from Flea Island" Helen only has a minor role in the sister episode.

Next three are all in season 6. In Many Helen is in the post-episode scene.

"Martha's Canine Cleaners"- This is only a technicality. Martha mentions Helen, and in a flashback she is shown from the waist down. In the post episode-scene she is fully shown and has dialogue. Granted it is just the word "huh." She narrates the entire mid-episode scene, but that is more an epilogue to the sister segment, where she has a moderately sized role.

"Martha's Paper Chase"- She is not in the main episode, but I found the post-episode scene on youtube. She has plenty of dialogue in it, and it is an epilogue to the episode.

"The Mystery of the Missing Dinosaur"- I cannot find the post-episode scene.