Welcome to the Q&A Forum

Couple of things of the top of my head.

• Check your rankings to make sure that nothing has changed (sorry if that is a no s***)
• Check the Google trends for your keyword to see if it is on the decline or not. You can check on either google.com/trends or google keyword tool but I totally blanked on how to do it.
• Has anything like bounce rate/site speed or down time change? These could account for the issue

The general rule is 1 per page. If you were going after "red widgets" make a page for red widgets add content for red widgets but please do not talk about blue widgets. If you do this Google will know that you talk about blue and red widgets but not which one you want to rank for. Additionally, try to have different pages for terms like "red widgets" and "red widget reviews" these two terms are very similar but they should be categories or separate pages from each other instead of on the same page.