As you assemble and tweak your initial key- frame you may need to adjust the z-order of your Sprites.  (the order in which they are drawn on other words, which are in front and which are behind). This can be done by clicking and dragging on the sprites in the z-order palette on the upper-left of your screen or by selecting a sprite on the canvas and then holding Cntrl and pressing the up or down arrows.  You can also hold Cntrl and press the left or right arrows to send the selected sprite to the absolute top or bottom of the z-order respectively.
Adjusting the Z Order of Sprites

Spriter Pro User’s Manual version 1.4

It’s very important to remember that Spriter allows for the z-order of all images to be diferent at any point on the, if you change the z-order on one key-frame and would like the new z-order to effect the entire animation you must be sure you’re on the keyframe with the desired z-order and then choose “Edit/Copy Z-order To All Keyframes” from Spriter’s menu.