Are you certain that it is showing up in searches? Use "Incognito Mode" in chrome and do a search of your area to confirm that you are still listed. From the city search, count how many search pages you are behind. Then go to the map fully zoomed out to the city limits and see how much you have to zoom in before you appear.
Otherwise, are you sure that you are keeping up with your competition? You may have a good backlog of reviews but it doesn't take too many savvy hosts with deep pockets to outcompete you and bury your listing. Assuming good management and competitive pricing, your only option is to step up your listing (better furnishings, decorations, professional photos etc). If you are comfortable posting your listing I'm sure you would get some helpful feedback.
Also, you may consider VRBO. VRBO seems to be bigger in the "legacy" vacation markets that predate OTAs, which sounds like your market is, while AirBnb is more dominant in urban areas and college towns. Either way it's worth a look from the sound of your situation. Good luck.