A null value in a field is most often the result of an attempt to write a non-numeric value into a numeric field.
In your case you can check the look up table created using model #1 and see, based on your example, a valid numeric value.
If model #2 is simply looking up the result of model #1's output (and that is know to be OK) then assuming the field types match and so on (is model #1 output to a text file or Excel or something else?) there woould seem to be little reason for the lookup data to result in a NULL field UNLESS for some reason that particular line has no match on the key(s) (And the "EMP TX ext lkup" is a numeric field).
Sometimes the keys may look the same but for some reason are different - a leading space for example or something like that.
Have you checked through step by step? Try something like adding the key fields to the lookup as well - if they come through blank you have a mismatch.