you have to have enough vehicles on all lines to trigger the production. 2 carriages/trucks won’t do. More vehicles means lower waiting times.
There is a hard coded time limit of 20 minutes before each good crate must reach its destination. That includes time spent from the mine to the station, waiting at station, unloading, moving it to the second loading bay, waiting for a truck there (THIS is what is killing your line, btw), loading etc, etc…..
When doing complex lines like that (transferring goods to a second line), you must take into account all the waiting times possible.