When you select a mesh as an emitter, there's a toggle for "Animated Shape." When this is unchecked, the emitter is only evaluated at the first frame, primarily to save processing cycles and because with a non-deforming emitter it's completely unnecessary. When you check the "Animated Shape" button, the emitter is evaluated at each integration step to account for changes. This is slower, but necessary for deforming geometry.
This is also true for animated textures. You have to force the PositionObject operator to reevaluate the emitter mesh at each integration step as the texture is changing.