There's nothing problematic about opening a lookup to 80,000 rows, as long as it's in a data format that can handle that (unlike .xls).
What version of Monarch are you using, what happens when you click on Refresh Database and External Lookups, and is the lookup on a network location or local?
It's a delimited text file. I'm using Monarch Pro 10.5. Refresh Database and External Lookups doesn't fix it.
It is linking on a character field that contains numbers. One thing I will say is that I put a row of "A"'s at the top of the file. This is because, without it, Monarch would try to match the column as a number even though the numbers were in quotes. Any advice would be appreciated.
Are the linked fields actually coming up as NULLs or blanks?
If nulls it would suggest that the receiving field is defined as a numeric and that it is receiving Character data - hence the NULL.
If it's character field receiving characters and yet it's blank blank then there is something else going on but I would suggest going back to basics and re-checking you definitions as a first step. Many is the time that the simple thing that could not possibly be the problem, er, was in fact the problem. Sometimes modelling is like proof-reading - very difficult to spot the glitches in something you have created.
If neither of the above ... then we will to think up a new question or two.