Get field value of sub site in wordpress by ACF

We have a Wordpress website which has 10 sub sites(blogs) . We show the latest post of those sub sites on the main site . Recently we added and use Advanced Custom Field which is active now on the main site and all those 10 sub sites and in each one we sat just a true/false field and its name is : is_announce . In each site ( main or sub sites ) we have access to that field by get_field and it works great . The problem is , on the main site we have no access to those fields of sub sites.It seems get_field works good just for each site's scope ( main or sub sites )

How can we fix it ?

Addition : As i told you we show the latest post of each sub website . each one has been featured by one custom field (is_announce) . We want to read its value and if it is True , show something on the top of each post , something like tag .

via Chebli Mohamed