Why does backslash return a solution of all zeros to a non-singular system of equations?

I am working on a project that uses a finite difference model. I use the backslash operator to solve the system of linear equations (solve for x in Ax = b) for a large, sparse matrix A. When I use the size of A that I want, the solution that results is all zeros, even though this should not be possible due to boundary conditions that I set. If I make A smaller, the solution makes sense. Is it possible that the matrix is too large to solve? I have tried many sizes for A, and there seems to be a cutoff point where MATLAB returns all zeros. Using the large size for A is important to the project.