usefulness of 'solve' matlab function

Hi, I would like to know if someone tried to use the matlab function 'solve', for example syms x; solve (F(x),x); when F(x) contains a parameter which changes its value, but the function 'solve' gives me a result for some values of this parameter , and 'empty' for other values... Is'nt the function 'solve' smart and useful whatever are the parameter values in F(x)????