At Hollander it was found that Carbonite was skipping a few files. The one noticed was a .png graphics file (looks like a scan or screen shot of a web page.
Discussions with Carbonite technical support got to the fact that this file was set with a T (temporary) file attribute. Because of this Carbonite would not give an option to force backup of this file.
I found a utility called BulkFileChanger at NirSoft.net. This enabled me to sort files by attributes to find ones with the T attribute. Out of 7,000 or so files in the Shared folder on this server there were only about 5 or 6 files marked with T and I changed these so now they will be backed up.