[firedrake] Divergent nonlinear solve

Dear Firedrakers, I've encountered a problem recently in trying to solve a particular PDE. This problem is elliptic in five dimensions, and I am working on a spherical shell mesh with outer radius 1. In Gmsh I've set the element size factor to be 0.05 and this works just fine, however when I refine the mesh to a size factor of 0.02, this results in the "nonlinear solver failing to converge after 0 nonlinear iterations" with the reason of DIVERGED_LINEAR_SOLVE. I find this a bit confusing especially since the problem I'm solving is linear. Any advice on how to solve this or why it might be happening? I'm using a ksp type of "cg" and a pc type of "gamg." Maybe I should try something different? Andrew Hicks