Oxidation can definitely play a part.
My Marshall LEAD 100 Mosfet is known for this and it happened to mine too.
The way to test that is to insert just a patch cable in the fx-loop. By inserting the cable, the contact that the Cliffs are making with themselves (where there's oxidation hampering the signal) will be broken by the inserted jack plugs.
Spraying some Deoxit or Contact Spray on jack plugs and repeatedly inserting them in the fx-loop jacks can sometimes overcome this problem. Don't use WD40!!!
Another -worse- possibility- is when the Cliff jacks are PCB mounted and there's a break in the solder joints. Since the jacks are a point of mechanical stress -by inserting and removing cables-, a cold/bad solder joint could worsen after x time of use. If this is the case and you're not handy with a soldering iron -nor accustomed to working in a high voltage circuit-, leave this to a qualified tech.
But step one would be to see if you can clean the contacts of the jacks, maybe with a Q-tip and some Deoxit or contact spray.
Oh, and make sure you use decent Qtips, so that the fluffy tip doesn't end up in your jack and you have to use tweezers to get it out... That totally never happened to me.
