A message has a yellow symbol beside it, but it does not appear in the list you expected. In Gmail, a star and an importance marker are different controls. They can overlap: Gmail uses starring among its signals for predicting importance, but the two lists are not interchangeable.
Choose the mark you actually need
For a message you want to find again, click its star in Gmail on a computer. Open Starred on the left, or search for is:starred. If the sidebar section is hidden, expand More. These steps find a mark you applied; they do not schedule a reminder.
The importance marker can be filled yellow or empty. Gmail predicts importance using signals including whom you correspond with, messages you open or reply to, keywords and actions such as starring or archiving. Hover over the marker to see an explanation where available. Click it to correct a mistaken classification.
Compare two searches
Use a nonprivate message you can recognize. Note whether it has a star, an importance marker, both or neither. Search is:starred, then separately is:important. Check the actual message rather than assuming that the same yellow color means the same status. Conversation view can contain several messages, so open the thread when the row is unclear.
For an ordinary follow-up queue, agree on one practice: for example, star messages that still need your reply, then remove the star after responding. An importance prediction alone cannot tell whether you finished the task.
Hiding markers is not clearing the list
In desktop Gmail, Settings → See all settings → Inbox contains importance-marker options. You can hide the markers or stop using past actions to predict importance. These are separate choices. Hiding a symbol should not be treated as a record that all previously important messages have been reviewed.
Before changing the display, save the search you use and check it afterwards. If what you need is a message returning at a particular time, use the separate Snooze workflow rather than expecting either yellow mark to behave like a timer.