Hello,

1. Try to disable/enable plugin one by one to check for a possible conflict. Disable all the performance/optimization plugin and check it is working or not.
2. Update PHP version if it is not PHP 7.
3. Increase WordPress memory limit to 256M.

Also, add the below code the Customize > Custom CSS section and check it works or not

h1.product_title.entry-title {
    display: block !important;
}