Depends on how your action is set up. I have one that puts it in the lower-left corner and nudges it a bit from the bottom and left side. Centering is a problem because the action only remembers the number of pixels from the top and left corners. If the images are different sizes, it will always put them in the wrong place. To fix it, do the following:
record a new action:
open your image, copy and paste your watermark, make your blending changes, then, in the layers palette, select the background layer and the watermark, now, with the Move Tool (V) selected, use the Align Right, Left, or Center and then Align Top, Bottom, or Center, etc. depending on where you want it all to be (dead center is Center/Center). This way, the watermark starts in the same position each time relative to the image, regardless of image size, then select the water mark and nudge the watermark away from the edge(s) as needed. Remember to stop recording the action, too.
If this isn't working or you need something else done, email back. :)