I use Nestle's Everyday Whitener to make coffee. But with some brands of coffee (eg: Davidoff) the creamer doesn't seem to dissolve. It instead remains suspended as granules in the coffee. But with other Nescafe brands of coffee the dissolution occurs easily. What's the difference and how do I fix it?
It depends on the contents of the coffee and the creamer and how they react to each other. For example some creamers are just corn syrup solids mixed with emulsifiers, and they usually mix less effectively if the coffee isn't hot enough.
Or, as you mentioned, if the problem is with the brand of the coffee, then it's possible that the coffee that's giving you trouble contains more oil than the other.
If you've soaked oily dishes in detergent, sometimes if there's enough oil, once the water cools down, there will be bits of solid oil floating on the surface. It's basically the same with your coffee. The warmer the temperature and the less oil, makes a better mix.
That said, I prefer using milk if you can. If you're lactose intolerant or allergic to milk fats, I'd suggest substitutes like soy, rice, or almond milks. The closest I've found is half almond, half coconut milk. Or better still, just drink it black.
Your whitener may have the same problem as when I use soy milk in my coffee. With certain brands of coffee the milk curdles due to the acidity of the coffee. I have found that the curdling is also influenced by the temperature difference between the milk and the coffee.
For this reason I suggest:
Buying coffee that you know is not too acidic for your whitener or at least heating the whitener before mixing it with the coffee.