Block select not selecting

Absolute newcomer. Using Magic Mouse set up to emulate 3 button mouse and also using Better Touch Tool to set up Middle Mouse Button. – Problem is when I use B and LMB I can see the box spreading over the area I’m selecting, but the minute I let go of the mouse button it disappears. – I click B and the tool appears; I select what I want dragging LMB; let go of LMB, and the selection disappears. Thanks