Phase fluctuations between the infrared and green beams during seeding of second-harmonic generation (SHG) in optical fibers lead to a drastic reduction of the initial SHG intensity and to a strong time dependence of the SHG after seeding. Experiments using mode-locked lasers with a stable phase relation between the infrared and green writing beams lead to reliable, reproducible results. Under these conditions we do not observe a significant difference in SH efficiency between fibers with Ge concentrations of 3 and 14 mol. %.