Hi Jacqueline,
 
I do not speak for GCU, only from experience. If too many declines was the reason, then your work must not have been meeting the Submission Guidelines, so take some time to go improve your craft. Everything you want to know about growing as an artist - in any form - is available online these days. Explore, polish your skills and then who knows ...
 
For a professional looking greeting card, it goes well beyond imagery. Design layout, typography skills and verse writing are equally critical.  
 
Good luck to you in your artistic endeavors.