But my dynamic lookup is not retrieving anything

If the information entered in the field used as the key for a dynamic lookup, cannot be matched to the key of the file being looked up the dynamic lookup will retrieve nothing and the fields defined to be filled by the dynamic lookup will not have anything put in them.

When a dynamic lookup populates the fields defined for it, the data it provides overwrites anything already in that field/s. Beware, this means that if you have previously done a lookup, and you do another where the key does not retrieve data for that field, the values from the previous lookup will remain in those fields.