Most private label brands that struggle picked the wrong product, not the wrong supplier. Research is where the money is saved.
We look at each idea the way a buyer and an accountant would. Is there steady demand all year? How many established sellers are there, and how many reviews do they have? What will the product cost once it reaches an Amazon warehouse and the FBA fees come off?
Then we read the reviews of the top competitors. The complaints buyers repeat become the improvements your version should carry, and the brief for the supplier and the listing.