Why is my ad not appearing on the first page?
The search ranking of organic ads, or those posted without Sponsor Ad, are expected to drop over time as newer ads in the same category, or of similar nature, are posted by other sellers. This is normal behavior for ALL advertisements, don't worry.
How do I make sure my ad appears when searched?
Though an organic ad will go down in ranking the older they get, relevance of keywords used still plays a major role on how it can be searched by your viewers. Using keywords that are directly related to your item helps the ad to become more searchable. Better yet, think of keywords that you think will most likely be used by your viewers when searching for your ad.
So, what can I do to bump it up and make it appear on top again?
Given that you've followed the tips, and your ad has gone down in the rankings naturally due to its age, you may consider using Express Refresh to bump your ad up again in the search results and extend its lifetime for another 28 days.
Another route you can take is Sponsored Ad. This will place your ad either as the first or last ad of search results pages making your it consistently visible despite its age. Learn more about Sponsored Ads here.