Time to degas, Ms. Cheeto?
Most of my non-malt based brews (ie wine, hard cider, hard lemonade, etc.) slow WAAAYY down after about 5 days unless I degas it every day. If it's in a bucket, just pop the lid and stir the crap out of it for 20 seconds or so (with a sanitized spoon, of course). Your yeast may be experiencing CO2 toxicity and are unable to work efficiently because of it.