My pages are falling out of the Google index

I've been really working hard and waiting about 5 months to index at least half of my pages on google. A week ago after typing in google "site:mysite.com" I could see about 15% more pages from my website than I can see today, the trend is downwards and it has been 4th day of checking. Is it normal for that amount of posts to fall out of the index within only a few days?

What can I do about it?