Why is my fragrance oil sinking to the bottom of the warming dish?

I have a fragrance-oil burner, where you put the water and the oil in the dish on top then put a candle under it. Like this one:


http://www.bathandbodyworks.com/product/鈥?/a>





Anyway, so I filled the dish with water and put some drops of oil in it, and the oil sank to the bottom of the dish! This is the first time I've had this happen. The oil is supposed to float on top of the water, but I don't know what is causing it to sink. Is it expired or something?Why is my fragrance oil sinking to the bottom of the warming dish?
No its not expired but I would say that use less water and more oil, better still get a fragrant candle on top and use a tea light down the bottom because they really last much longer and permeate the house quicker the store I buy from is called Dusk, dont know if you have a store like that but they really are worth the money! :)