Is that all of the devices you have. One dimmer, one lock and the stick. (I feel like there is a song lyric in there somewhere, haha)?
When you look at the zwave device, does it show each device as a neighbor to the others? in the arrtibutes? In a perfect world the lock should see both the stick and the dimmer as a neighbor, but if the stick is too weak or far away, then as a minimum it should see the dimmer.
The issues I tend to notice on these posts, is that when you only have a few devices, if any one of them is not available for any amount of time, there is no where else to “hop”.
When it is working, does the lock fully work? You can lock/unlock, it reports its state properly? You can change codes, set zwave parameters?
With that said, the FE599 is a bit tricky and slightly different than other locks. Have you added the lock to group 2 node to control