Varnish is a high speed caching system that some hosting companies employ to keep your site running very fast. The problem with varnish is that it also can cause eCommerce systems trouble with dynamic content.
WP EasyCart relies on a site’s dynamic abilities to keep shopping cart contents full, users logged in, and track data as users move around your site. Unlike a static web site that just serves pages over and over again, which can greatly benefit from server caching, eCommerce such as WP EasyCart can really have issues with these processes.
Many hosting companies allow you to disable varnish or at least exclude certain URL’s from caching. Other hosts require you to ask them about excluding or disabling, either via chat or their hosting support channels.
Often a quick google search for your web host will reveal ways you can disable varnish or build exclusions… such as this section for cloudways hosting.
Exclusion guide for Cloudways hosting: https://support.cloudways.com/how-to-exclude-url-from-varnish/
The typical exclusions you will want are for your /store, /cart, and /account pages of your website. Be sure to check the exact URL you have for those pages as they may be unique to your site, but excluding the store, cart, and account from varnish is a great way to insure it is not causing issues.
Sometimes it can be hard to see what is happening on your site with a dynamic eCommerce system. Here are some common things that may indicate you have varnish or other caching issues occurring.
There are other things that caching can cause symptoms on, but if you make a change, it should happen instantly on your front end WordPress site, if not, caching is involved somewhere.
For other documentation on caching, check here