When hoppers are used on locked chests by default they won't be able to function. This is because of the container locking system that is in place on the server. This guide applies to any container (block that stores items) that is locked.
To get around this the container locking system has flags which you can see a list of by using the command '/lwc flag'
To allow a hopper to function correctly with a locked container the flags that we need to focus on are 'hopperin' and 'hopperout'
Hopperin (Hoppers putting items into locked chests, furnaces and shulkers)
To allow a hopper to put items into a locked chest you need to turn on the 'hopperin' flag. You do this by using the command '/lwc flag hopperin on' and left clicking the chest that you want the hopper to work with.
Hopperout (Hoppers taking items out of locked chests, furnaces and shulkers)
To allow a hopper to take items out of a locked chest you need to turn on 'hopperout' flag. You do this by using the command '/lwc flag hopperout on' and left clicking the chest that you want the hopper to work with.
You can use both of these flags on the same container at the same time, so that you can make contraptions that use hoppers to put in and take out of chests.
You can disable these flags by using the same command but replacing 'on' with 'off'.
These flags can be applied to any container that is locked.