Why aren't you digging it? Use pinpoint mode to center whatever it is, dig it up, look at it, and your question will be answered.
There's nothing better than trial and error, digging 100's of targets, to familiarize yourself with what all the different sounds are telling you. A one-way signal can mean many things, depending on the machine. In some cases, it can mean that something that is disc'd out (a nail for example, that you're rejecting) is lying to one side of the object. So that when you sweep one way, the nail is in that line of sweep. But when you sweep the opposite way (90*) the nail is no longer in the sweep path to "mask" the good item. But that's just one example of an explanation. There are other explanations, unique to each machine, that no amount of text exchange will dicipher. Other explanations could be that you're only centered correctly over the item when sweeping one way, but not the other (ie.: you're not "x-ing" the spot well enough, to get the highest center reading, when going the other way).
Try playing with centering the target from many angles, varying your sweep speed (and making note of the various reponses), then ultimately, dig the target and see. Then the next time, you'll have better ideas what one-way signals mean on your machine. Typically it's better to have a target that beeps 360*. But of course, that may only mean it's shallow and easy. Perhaps a deeper one only hits well from a single direction (or only one way). Experience is your best teacher, and there's no substitute for hard-knocks trial and error. Eventually you'll get to where you recognize what to pass and what to chase.