Well, Maybe you could be more specific as of the problem?
I never used the effect loop when I had a 5150 but does the 5150 have a footswitchable effect loop? If you dont have a footswitch connected it could be that it's automaticly bypassed....just a guess. My JSX acted that why i think when I didnt have the footswitch connected...eather always on or always off...cant remember and there is no switch on the amp to turn it on, so you pretty much need the FS.
wild guess...im just assuming there's no signal going into the 2101 ?