leoncostelloe, it's going to be extremely difficult for anyone on a text-printed forum like this, to diagnose what you're doing wrong. Because ANY machine, set to "all metal" and "full sensitivity" (as you say) should be able to find REAMS of targets in any given empty lot, park, etc... I mean, at least some nails, foil, modern clad, etc.... The only exception would be if your hunt zone was absolutely sterile (no humans ever went there, nothing ever occured there, etc...), or a relicky site that others plundered the h*ck out of already (but even THEN, they usually don't dig all the iron, so if you were in "all metal", you should have AT LEAST gotten some nails).
So this tells me you're doing something wrong if you found "nothing" (and by "nothing", I assume you meant no targets at all, not just "no coins" etc.., right?).
And since detectors are "sound specific", it's too hard, in printed text, to diagnose what you're doing, what sounds to listen for, how to swing, etc... Because, afterall, "sounds" can not be described in printed text. You know, like if someone tells you to describe the sound of c-minor, in printed text, it can't be done. It has to be heard. Same for how to swing (speed, distance from the ground, wiggling and/or criss-crossing a target, etc...), how to isolate targets, etc. Those things can only be "shown", not read about in printed text. So you'll have to hook up with a proficient user in your area. Trade off flagged signals. Watch how he does it. Listen to what he's listening for, etc.... Start with an area with lots of easy clad (because good hunters don't mess with modern-clad parks). Or yards of 1940s/50s homes (if you have friends/relatives with such yards), that will have ample signals to choose from.
Don't rush out to exotic ruins/relics spots first (even though you probably detest clad like the rest of us), because if you start with hard-worked stingy spots, you will NEVER learn your machine. So start with something flush with clad, turn down the disc, force yourself to dig 1000 signals. There's no skipping this school of hard knocks. And THEN progress up to difficult stingy spots you might be researching.