There will be a point in time, that no matter what you do, your product monthly sales do not seem to be improving, or even declining.
The cause of this could be related to multiple factors, including but not limited to:
The product keywords CPC cost became very high and there is no way you can compete.
Your organic keywords are slipping away and it's a little too late to come back.
There are better and cheaper products than the ones you sell.
The product you sell is old (iPhone 1 case) :)
You are competing against well-known brands, and it's not your time "yet"
When this happens, you have 2 choices:
Trying to optimize every last thing by spending your week playing with individual keywords to sell an extra few units per month.
Keep launching stuff hopefully selling hundreds per month.
Once you see the bigger picture, and after a year or two, looking back, you will realize that you would've been better just to get it 70-90% of the way there, forget about it, do maintenance every now and then, and keep launching stuff.
When launching a new product, it's not going to take off immediately, rather it's going to be a long process, but the positive side is that it is going to be a long-term investment into your growing business that may pay off very well in the long run.
Follow strict rules for all the new products that you launch on optimizing, record the results, try A/B testing, do periodic maintenance, then move on to the next one.
This is the key to maintaining steady growth, you can't win them all, accept the fact that there will be failed products, products that do not sell as many, and steady winners.
Things to Consider
Do not sell anything you find, From the perspective of advertising, you're better off focusing on the same category. That way you can do better with your ads.
Diversify! do not put all eggs in one basket.
When you have a hit product, start creating variations, It will be much easier than a brand new product. Stay focused on your niche and brand, getting 5-10% more in sales on a current product is much harder than launching a new optimized product.
Dedicate time to research, and subscribe to a related newsletter or a youtube related channel. Offer the best quality product with differentiation at the best price and boom.