It would help if you could be more specific and especially what conclusions you have reached after comparing your results with Google's detailed Help file.
Your data show that over the last 52 weeks Google has analysed and decided your pages:
1. Total Indexed figure has gradually dropped from 150 down to 137
2. Ever Crawled has slowly risen from 39,096 to 39,237
3. Not Selected has gradually increased from 50 to 114
4. Blocked by Robots.txt had a slight increase from 0 to 4 for 15 weeks starting April 2011
My Conclusions and Two Cents:
Your site does not appear to be updated (1) and your content has decreased (3) so I assume that Google's Updates have discovered duplicate content and reduced your search exposure.
I believe that Google are trying to encourage fast, optimised, semantic error free sites with little or no missing files. Also these preferred factors are not critical and have a low rate of importance. (I am still amazed at the errors on some leading sites). Unique content and popularity is their main ranking factor.
Your site looks as though the decline will persist and desperately requires additional unique content to regain your previous popularity. Have you considered a forum or create your own suggestions in a "Question and Answer" section?
Last edited by John_Betong; Nov 1, 2012 at 00:33.
Reason: included missing results
Times change and it looks as though your site has been eventually suffered a Google Update.
I think your 20,000 impressions were gleaned from GWT -> Traffic -> Search Queries but far more meaningful statics are available at GWT -> Health -> Index Status.
Take a long look at the Help file at the bottom left, preferably use a print out and see over the past year how your data links have evolved.
GWT Help Excerpt
Recently I read an interesting article, that went through the stages of increasing the impressions of his friend's site. What struck me the most from this SEO Professional was the analysis, attention to detail, improvements but most of all the non-dramatic traffic increase over a six-month period!
There is no easy solution, it takes hard-work to maintain a high level of traffic because you are unable to influence other important external factors.