Hi There,
This may be an impedence mismatch problem. I suggest adding a buffer as the first item in the effects loop chain. This website has a good article on the kind of things impedance mismatched cause, as well as some good buffer suggestions:
http://screaminfx.com/tech/why-and-when-to-use-a-guitar-buffer-pedal.htm
Although not addressing an FX loop directly, the problems and solutions can be applied to the loop. The math is the same, and should work for you.
I use my H9 in the FX loop of my POD HD500X, and it is awesome!!! And if you really want to see what your H9 is capable of, you have to try it in stereo … you will Blow Your Audience's Mind!!!
Best of Luck to you!!!
