A little disappointed

I bought a difusser starter kit, that came with 2 essential oils, lavender with coconut oil and peppermint. I bought an extra bottle of essential oil grapefruit. Love the smell of these Simply Essential products but find the scents do not last long in the difusser, by 2 hours of it running i'm having to add more drops. I used 10 drops thinking that would be good and it is for the first 2 hours and have to add another 10 drops. The price was cheap 11 dollars at Walmart which I liked but I think paying more for better quality of essential oil maybe the scents would last longer. I think that's the route i'm going to try....A little disappointed with these brands.