I would greatly appreciate it if you could share some reasons the Conjugate Gradient iteration for Ax = b does not converge? My matrix A is symmetric positive definite. Thank you so much!Edit with more information:My matrix is the reduced Hessian in the optimization algorithms for problems with simple constraints. The matrix is 50% sparse and I am using matlab. 
What are some reasons that Conjugate Gradient iteration does not converge?
iterative method;convergence;conjugate gradient