Typically the Backward Links function only shows links with a PR of 4
or more. So whenever a page linking to yours loses PageRank and falls
below 4, it won't be displayed, and the value of that link diminishes.
You can see the PageRank of any page by installing the Google Toolbar:
Look at this page as an example of a one that links to your site:
It currently has a PageRank of 2. Other pages within that site are PR4
or PR5, except for their other links page, which has PR0:
Google software has decided that these links pages are not worthy of
PR4 and given them a lower ranking. Each probably originally had PR4
(unfortunately there is no way of finding out historical PageRank). If
the page linking to your site used to have PR4, then it would've
helped to give your site a higher PageRank, and it would have appeared
when you checked for Backward Links.
With time Google works out which links are reciprocal, and which pages
of links are purely there to influence PageRank. And with time Google
decreases the PageRank of those pages.
Last November (the Florida Update), Google began to remove the value
of links for searches involving commercial keywords, when the same
text was repeatedly used. Basically they made the presumption that if
the same link text was used for a large number of links pointing to a
page, they might not be genuine.
By asking reciprocating sites to use the text you provided:
Comprehensive (HGH) human growth hormone and IGF-1 information
HGH-best-Results.com provides news, articles, and information on HGH
and Anti-Aging remedies
...the old Google algorithm would have given your site a boost for
searches involving any of the words in the link text - because link
text is one of the factors Google looks at. But now it is probable
that, since last November, the value of those links has disappeared,
for any searches involving commercial search terms. The new SEO best
practice is to try and have more variety in link texts. My guess is
make sure that links using the same text should account for less than
25% of incoming links.
Reciprocal links may work in the short-term, but the ideal links to
have are genuine ones. Google specifically mention reciprocal links in
"Don't participate in link schemes designed to increase your site's
ranking or PageRank. In particular, avoid links to web spammers or
"bad neighborhoods" on the web as your own ranking may be affected
adversely by those links. "
Getting genuine links is much, much harder. The only real shortcut is
to pay sites with high PageRank to link to you, or devise very
elaborate linking schemes involving dozens of your own sites.
One way of looking at it is that - purely in terms of the Google
algorithm - you were fortunate to receive a high ranking in search
results previously, and now your ranking is normal, based on what
Google looks at.