Failure to get compactPDF to compact a pdf file

I am failing to get compactPDF to make any change to a pdf file
that, a/c to the message from the CRAN upload site, can be very
substantially compacted.  Any ideas what may be wrong?

I have also tried recreating the pdf file.  I also tried
R CMD build --resave-data --compact-vignettes DAAG

The data files compact alright (but I get the 'significantly better compression'
warning message that might suggest that this is not happening), but the pdf
file appears to go into the package unmodified.

<<<<

tools::compactPDF('/Users/johnm/packages/DAAG/inst/doc/', gs_quality = "ebook")
dir('/Users/johnm/packages/DAAG/inst/doc/')

[1] "rockArt.pdf"

sessionInfo()

R version 2.14.1 (2011-12-22)
Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)

locale:
[1] C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] tools_2.14.1