Are the LXDE and KDE installations using the same kernel? If not, maybe you need a kernel upgrade on the KDE. As Just18 says, check the driver set up in PCC ->Hardware. Finally, if all else fails (and if your wireless connection where you are can cope with the download size), you could install task-kde into your LXDE version which would enable you to use KDE there!
That last one shouldn't really be necessary, as there's no real difference between desktop versions other than the desktops.