Where are you clicking. If you Command + click on a layer's content icon in the layers palette. That layer pixels with more then 50% opacity should be selected or you should get a warning no pixels selected?
thank you for your answer my problem was resolved by chance today, i don't know since when photoshop changed its shortcut of selecting layers. i used to select the target layers by press "command" + left click right on the objects on the main panel. but it was until today that i found no needs to press " command" anymore. just a single click on the objects, then i can move it. thank you again