I don't know If this is your problem. But if you want Mathematica to solve the problem numerically I think you should tell Mathematica at which point you want the solution. As example:
If you had to solve f''(x)=-f(x), then the solution is f(x)=k*sin(x) (for every k). But, as far as I know, if Mathematica is not able to solve such an differential equation explicitly, then it can only solve such problems for special values of x (numerically), for example x=5, where the result would be k*sin(5).
Perhaps you have to tell Mathematica where you want the solution.