Fully understand, you’ll find a link on how to mask sensitive data in Post #3.
A Merge (without Fuzzy options) performs an exact match lookup and can only return a value when it is present in the lookup table - so if there are no 6 or 5 digit PC in that lookup table, the result will be null.
If you know the pattern of missing digits in the PC you could add those variations to your lookup table. But there can also be an issue with the text value itself like additional spaces, unprintable chars and so on, you’ll have to examine that as well.
This is the best I can do for you without any data.