Getting hit with a penalty on your site can be an insidious thing. One that you frequently won’t even know about. There ARE the obvious ones that show up in your Google consoles – the dreaded Manual penalties. Unnatural linking, Pure Spam, etc. These are going to show up readily if you go hunting for penalties. Much more often will be penalties associated with over-optimizations – keyword densities too high, urls that are over-optimized and other penalties that don’t incur any real wrath, just creating poor rankings.
These types of penalties can also be applied as time goes along. That is a thing that always needs to be considered. Google algorithms are NOT an “etched in stone” thing. What was great LAST year, and maybe even REQUIRED to get the best rankings, may have fallen completely out of favor with the Google algorithms THIS year. High keyword densities were once pretty much mandatory to get awesome rankings at one point. While publicly declaring that content should be written for the readers and NOT for the bots, the bots/algorithms HANDSOMELY rewarded over-optimzed/high keyword density content. Finally the algo started penalizing the high keyword density pages. So if you had content that WAS once ranking well, check the algo changes.