why does my non dairy creamer not dissolve in coffee?
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.