I have this problem too and it's not about plurals. It's also nothing you are doing wrong and there is no way to correct it that I have found. Adding fresh content everyday will not help either. I experimented with making massive changes daily and doing nothing at all for weeks. I think it is google using different algos. It seems the longer your page has been in existance the less likely this is to happen.

The best way around this is to find another product to sell or even 10 more products to sell. Build 10 different websites selling 10 different things. It's a lot of work initially but the only way to be in this business is to make way more money than you need to pay the bills.

People who earn their living off of internet search engines should be sure to work hard enough to make at least 3 times more money than they need. Bank the rest for the rainy days that are sure to come.