When I do a data merge into CS4 from Excel 2010, some of the data merge fields don't get populated. The problem is random. For example, on some records the gross total will have a number in it, and on other records the merge field <<gross>> appears.
Also, I have noticed that the gross total for an individual will appear on the next record and that all the records get out of sync. This will go on for several records, then correct itself, then get out of sync again. This issue happened with monetary amounts and people's names.
Are there compatibility issues you are aware of between CS4 and Office 2010 that affect data merges?
There have been a couple of other reports of this sort of trouble in CS4. Most of them were cured with patches, so have you updated to at least 6.0.5?
Other problems that were similar were traced to bad data files, so open your data file in a plain text editor and check for odd characters and the like, then resave.
And I've seen one data file that works in CS3 and does not work in CS4, and there is no reason anyone can discern.