When you create a lookup there is a line underneath the TO selectors that says "This field will be updated when a new value is inserted into ".....".
There are also a number of other options such as "don't copy if empty", and also for a not valid case.
When these are correct then it will always update according to the statement below the TO selectors.
It is worded differently but it is there under the to selectors. "when a new entry is...". That explains when the look up is updated.
Sorry but look ups do not update automatically when data changes in the table from which the data is being looked up.
You can script an update, but since you have a valid relationship already (or your lookup won't work), just add this field from orders to your assets layout in place of the one defined in assets.