Soulver has one of the smartest mathematical parsing engines in the world. It works hard to analyse your problem, and get you the answer you would expect.
While Soulver usually works great for many expressions, it's not magical. It won't be able to divine an answer from some complex expressions. Only specific words have mathematical meaning to Soulver, such as units, and percentage operations.
If Soulver is not giving you the answer you expect, try to express your problem in simpler mathematical terms. You might also find it useful to define a variable to represent a certain word.