A couple of things to check:
1) Make sure that you are dragging from near the left edge of the screen. It only detects a swipe starting from a certain region.
2) Make sure that your screen's hitTest() is not ignoring the touch. PanelScreen or ScrollScreen should be fine, but if you're using Screen, you might need to add a backgroundSkin (even if it's just a Quad with alpha set to 0).