Please post the URL to your gallery's web page so that I can take a look at the problem and hopefully help.
The problem may be to do with SimpleViewer or Showkase but without being able to see the problem, I do not know.
In a Showkase site where the size of the gallery is dynamic (depending on the size of the user's browser window), it might be wise to not use a Fixed Layout. Please try this to see if it helps.
Depending on the layout of your SimpleViewer gallery, you might also need to set floatCaption="FALSE" (in the 'Caption' section) to ensure that the height of the caption area is taken into account in your gallery's layout.
Also, you could try editing your SimpleViewer gallery page in Showkase, scrolling down to the 'Override Theme Settings' section and entering a positive number for 'External fit px' to increase the height of your gallery on your page. (Be sure to click 'Update' and 'Publish' afterwards.)
Hopefully one of these suggestions will help but, if not, please let me know the URL to your gallery so that I can take a look and help further.