The iterative solvers have a tendency to 'stall', that is to stop error after some numer of iterations. The problem is more prominent when the meshes are refined or if the meshes are very very fine. For many iterative methods the number of iteration required to converge the solution are linearly proportional to number of nodes in one direction. This behavior could be rooted out to the fact that during the iteration process, the information travels only one grid size per iteration. And for proper convergence the information has to travel back and forward several times.