Batcache not working with 2 Wordpress sites on the same server

Hi

I have 2 Wordpress sites on the same server with Batcache (Github version by Automattic) and APC Object Cache Backend plugin installed.

Unique WP_APC_KEY_SALT defined on each wordpress site.

Debug option turned on each site.

When I check the debug message: -
siteA works perfectly, Batcache message is shown;
siteB doesn't show anything about Batcache

No error has been logged in Nginx & php error logs.

Any suggestions?

Thanks!