When an element does not appear on a page where you would expect it to, the cause usually lies in the targeting chain between the element, the display rule and the position.
First check whether a display rule is actually linked to the element, and if so, whether the pattern type and the pattern of that rule match the URL of the page in question. A rule with the rule type ‘Exclude’ that has been set too broadly by mistake may hide an element on pages where it should just about be visible, whilst a pattern that is too strict for ‘Include’ may, conversely, hide an element on pages where it should appear.
Also check whether the display rule is restricted to certain languages, and whether that language restriction matches the language version of the page you are visiting. The position itself also warrants attention: is the position set to ‘Active’ status, is your element’s element type included in the list of permitted element types for that position, and has the embed script for that position been correctly implemented in the right place on the page?
If all these elements are found to be correct, check the campaign priority as well: where multiple campaigns target the same position, the priority value determines which campaign takes precedence, meaning that a campaign with a lower priority may sometimes be overshadowed by another.