WordPress does not use PHP sessions, that's why some hosting services might have issues with plugins that do use them.
How do you know that PHP sessions are causing problems on your site? First you will have to enable WordPress debugging on your site (set WP_DEBUG to true in wp-config.php file). Once you do that, you should see a warning like this:
Warning: session_start(): user session functions not defined
That will let you know that PHP sessions are not working on your site.
There is no single solution to fix everyone's problem, since this issue is dependent on the code that is using PHP sessions. The first thing you can try is to install this plugin: WordPress Native PHP Sessions, that should solve the majority of session problems. For more information on setting up this plugin, please visit this page. If that does not solve your issue, then please contact our support and we will take a closer look at this problem.
Additional information on PHP sessions and WordPress: