why everyday different URLs in SERP for a particular keyword for my website

[seopro11077]

 I have a website and for a particular keyword, google is showing different URLs for everyday in SERP.  Sometimes it fetches data from title-description, sometimes from alt tags and sometimes from content.
I have a specific page optimized for this particular keyword but it is not getting up in results due to this problem.
Can anybody help me why is this happening?