The Geograph search only retains you most recent 35 searches (unless you [save a search]). As you add more searches the oldest will drop off the list.
You can [save a search] by clicking the yellow hollow star to the left of it on the list - the star fills in and the search terms are then shown in bold. To [stop saving] an individual search, click the star again and the search will drop off the list like any other unsaved search.