Grant has already supplied some good advice in my absence (sorry for the delay, but I was without access to the site for a while).

I can’t recall a situation wherein we couldn’t get an external lookup working. Sometimes, as Grant noted, it requires a conversion field, constructed with a calculated field, to help out.

The external lookups should work just fine BUT, as with any interactions between systems – even Access and Excel in days gone by – data storage formats may not be the same underneath what you see on screen. This is most often (and frequently) true for numeric values.
So the first step to getting a lookup to work may be that you need to normalize the data. Monarch allows you define the link via a calculated field so as long as you know (or can guess) what the underlying data structure is in your external lookup source it should be possible to have Monarch bring in the linked data and re-format it in advance of the compare.

Typical problems often involve field lengths/number of decimal places on numeric links where the field definitions simply don’t match. I character fields things like leading/trailing spaces and even sometimes double and triple spaces where you are not expecting them (spaces are difficult to count) in a string can be a problem.

Remember that a lookup link is just something that is a code which, often, needs to be relatively unique. The key to success (apart from knowing you have reasonable data on both sides of the lookup!) is reducing everything to be as consistent as possible and so eliminate potential errors or mismatches that are built into the data. It you are using records from different systems running on different platforms there will be many situations where they will not have an exact technical match. Monarch makes it pretty easy to work around those limitations once you know what it can do.

Grant