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Reduce the Feather value in the Options bar before drawing with the Lasso Tool.
Also make sure your targeting the right layer. Selection are made on layers if the layer contains transparent pixels or the layers opacity is set low you will not see where the selection is for Photoshop only places marching ants around pixels that have at lease 50% opacity.
I was having the same problem and I figured out why. I was deleting the background layer. I do not understand why but the background layer needs to be there to select 100% of the pixels.