I'm not real familiar with the 5150, but does it have a knob on the back for "effects level", or something like that. My Marshall's have that.

Also, check the batteries in the effects pedals. Just b/c the light is on doesn't mean it's getting the power it may need.

One more thing, are you using stereo cables going in the loop? Not sure if this would cause the problem, just trying to think of things that may be causing it...Probably don't want to use stereo cables.