Most common problems for something like this:
1) Your subscription ran out! You can check in the "Account" tab, any expansion in color you have access to, anything in greyscalce you don't have. Ending dates in YYYY-MM-DD format are below the expansions for more detail.
2) Check that you are including rated games in your automatch settings. Unrated games are less popular in automatch.
3) Check that you have "Extra Menagerie" not checked. Those again are again less popular.
4) Did you significantly change the time you play? North America has AFAIK the biggest user base, so when players from there sleep or are at work/school, attendence will drop.
If 2) or 3) are violated, they are likely missclicks you did, as that didn't change recently. If none of that is true, I recommend posting a screenshot of your matchings tab here in a followup post, so I can look what else may be off.