Don't fret. This is typical for most things online. People like new stuff. As it gets a little dated, people are less interested.
So, here's what you can do.
Decide what's the best course of action for you.
1. If it's a good stand-alone product, then yes you might consider slowly testing some ads. Don't spend too much in the beginning or you can eat away your profits.
2. Consider value addons. How can you add to your product to increase the number of sales and/or increase your price?
3. Once you know you've got a good product, and some ads that work, and have added even more value, then slowly increasing your ad spend to scale it up can be a good idea.
But if it's just a one-off product and hard to addon, then I wouldn't spend much more time and money on it. I'd just move on and create your next product to sell. Don't wait around. Immediately start working on your next product. Even if you end up with several different products which don't sell that well anymore by themselves, you can still sell them later as a group, or membership or PLR.