I have a county-level map that is displaying nicely ... except for around 300 counties, all in 5-6 states, which are blank. All of the FIPs, county names, lats/longs are consistent in the data file, so I'm puzzled. Any clues?
In Excel, create a new column K, with formula
=B2 in cell K2, formatting user-defined to
00000. Copy the cell down to the last line.
Export the XLS as CSV, and do the join in carto on that new column.
Or set the FIPS column data type inside carto to integer, as explained in Georeferencing 1990 U.S. county names / FIPS for CartoDB?.
Looking through your xls data I noted all the missing counties have a four digit FIPS rather than a five digit FIPS. If you are linking this data to the county polygons FIPS excel could have truncated the initial "0" and you are not getting the needed information because the link fields do not match.
Try converting the excel FIPS column to text and/or adding a single quote in front of the numbers that start with zero. For example '01001 for Autauga County, AL not 1001.