Losing traffic from a Google update is depressing, especially if you feel you've upgraded your site and don't deserve a lower ranking. The first thing you have to do is determine where you've lost traffic so you can isolate the problem. If you don't do this, you will waste time on meaningless improvements and possibly miss a major problem.
First, check for seasonal issues and recent trends. Are your keywords receiving less search volume because your main product or service isn't popular anymore? If you sell GPS devices or MP3 players, people don't buy them very often today. Check to see if a recent innovation has rendered your products or services obsolete.
Check to see if Google or Bing are having problems crawling any of your pages. If multiple people can update your site, someone may accidentally added a no index directive to certain pages which you want search engines to index.
Check your analytics. Are you losing mobile traffic? Perhaps your site isn't as responsive as you believe.
Check analytics to determine the performance of individual pages as well to see if you've lost traffic to certain pages. If so, analyze these pages carefully. Look at the pages still getting a decent amount of traffic to see what you're doing right. Try to duplicate this on the pages where traffic is dropping.
Analyze your link profile. While unlikely, it's possible a competitor is creating low quality backlinks for you. If you have an SEO company which works on your site, they may use a private blog network or other black hat tactic you don't know about.
Perform some anonymous searches to see if competitors are moving up in the ranks. Maybe, you're doing everything right, but they have made major improvements to their site and now outrank you. Slipping down a few positions in the SERPs could account for a loss in traffic. For more information click here https://www.reddit.com/r/SEO/comments/ey5hz8/myprojectlost40korganicvisitorsmonthly/.