From my testing its because you have placed the crate on the ground (left clicked not dropped). As long as I haven't placed it, I put as many in the truck as I want, not just one. If I have placed it I can't carry it in the truck at all. Try swapping crates in the truck and see if its true for you? Then at least you'd only have to buy some more crates, not make a ton of trips.

Appears to be if crate was ever used, or perhaps when emptied, it will no longer "attach" to truck-bed. I had a few leftover that I hadn't used yet, and those stayed in the truck. I have quite a few that even though they are empty, the continue to list last known item w/ qty=0. Again, thinking maybe a glitch when emptied.

Originally posted by Elcadion:

Appears to be if crate was ever used, or perhaps when emptied, it will no longer "attach" to truck-bed. I had a few leftover that I hadn't used yet, and those stayed in the truck. I have quite a few that even though they are empty, the continue to list last known item w/ qty=0. Again, thinking maybe a glitch when emptied.

Im having the same issue. Left clicked to place down then used them. Decided to move them to a new location. I emptied them(some still show item name with 0 quanity) Tried to place in truck or othe storage crate but wouldnt go in crate and on truck when pulling off the storage container just stayed floating in the air. Gonna take along time to carry one at a time to scrap them. Hopefully a fix will come soon.

I've found a workaround:

• pick up the crate
  
• click to enter placement mode
  
• right click to exit placement mode
  
• The crate behaviour is now "reset" and can be placed on carts without collision, and keeps any stored items

This works for empty and full item crates, and can transport multiple crates on the cart