you know that everyday approximately thousands of website come to live if new one can give better contents than you and better optimization than you that means new one can getting higher ranking than you too. So in this case your keyword ranking are going down.

Always check your competitor keywords, use keyword planner and Google trends. Create quality back links this will help you a lot.

Select the forums, blogs, advertisement, bookmarking within your niche. make back links on several sites. The keyword you are optimizing should be included in url or our site
eg website url/keyword/ keyword phrase.

I have a group of keyword that constantly move within the Google SERP. One week they are in the top 3 spots the next they fall down to position.

write the best content related your business and choose keywords

Try these following steps to improve your keyword ranking
1. Social Bookmarking (High page rank sites only)
2. Blog commenting
3. Forum commenting
4. Directory submission (High page rank directory)
if you concentrate on these above topics keywords will reach its ranking automatically.

The reasons will be

If Site loading time is more.
Placing wrong keywords.
Your link building is behind your competitor’s link building.
If you are getting low quality links.
Loosing Good quality links.
Duplicate content.