the documentation here:
http://doc.ez.no/Extensions/eZ-Publish-extensions/eZ-Flow/eZ-Flow-2.5/Usage/Block-Management/Block-Management/Adding-content-to-block

states that items added to blocks are available to be published immediately. however, there is a "safety" introduced in https://github.com/ezsystems/ezflow/blame/master/packages/ezflow_extension/ezextension/ezflow/classes/ezflowoperations.php#L185
this offset will discard items added less than 5 seconds before the publishing.

test case:
1 open any frontpage object, select any manual block type
2 click the "add item" button
3 select the item to add
4 the queries run, and the item in question is added with timestamp X
5 the admin backend returns to the frontpage object edition
6 click the "send for publishing" button
7 the blocks are updated with the information in the ezm_pool at time stamp Y
8 open the frontpage: the item added is not in the block

note: the steps 3 to 6 must be performed in less than 5 seconds.
alternative test: increase the value in ezflowoperations.php#L185 to 60 seconds. now steps 3-6 can be performed in <60s to get the same result.