It sounds like you really need to get debugging working.
From what you said about the "interp num" command, I assume
you are building the fluidsynth command shell. Do you get any
relevant error messages on the console (probably the same
terminal you are running fluidsynth from - but may be hidden
if you are accessing the shell through TCP for example). Make
sure you are able to see console messages, since there may be
some useful error messages getting output.
When running the interp command it should
call fluid_synth_set_interp_method(). I would get debugging
working and set a breakpoint on that function and make sure it
is doing all the calls to fluid_channel_set_interp_method()
for every channel.