Equal height option of Advanced Post is a purely JavaScript dependent functionality, where we are calculating the post height on page load & applying that height through JavaScript.
Sometimes, this functionality fails to calculate the equal height of blog posts if the LazyLoad feature is enabled by any third party plugin or the theme that you are using.
Basically, Lazy-load is the functionality which loads images when image area comes in viewport. With this scenario – Advanced Post’s equal height option fails to calculate the exact height of blog posts.
In order to use the equal height option, regretfully you would need to disable the Lazy Load functionality.