stumped on re-building package vignette

[Env: Win Xp / StatET 2.0 / R 2.15.0]

In my heplots package I extended the HE-examples.Rnw vignette under 
inst/doc. The package passes R CMD check
on my machine:

* using log directory 'C:/eclipse-3.7/heplots.Rcheck'
* using R version 2.15.0 (2012-03-30)
* using platform: i386-pc-mingw32 (32-bit)
...
* checking sizes of PDF files under 'inst/doc' ... OK
* checking installed files from 'inst/doc' ... OK
* checking examples ... OK
* checking for unstated dependencies in vignettes ... OK
* checking package vignettes in 'inst/doc' ... OK
* checking running R code from vignettes ...
'HE-examples.Rnw' ... OK
'repeated.Rnw' ... OK
...

However, on R-Forge and on CRAN, the following error/warning is generated:

Mon Jun  4 20:18:22 2012: Checking package heplots (SVN revision 136) ...
* using log directory ?/mnt/building/build_2012-06-04-20-02/RF_PKG_CHECK/PKGS/heplots.Rcheck?
* using R version 2.15.0 Patched (2012-06-03 r59505)
* using platform: x86_64-unknown-linux-gnu (64-bit)
   ...
* checking for unstated dependencies in vignettes ... OK
* checking package vignettes in ?inst/doc? ... OK
* checking running R code from vignettes ...
    ?HE-examples.Rnw? ... [4s/4s] OK
    ?repeated.Rnw? ... [4s/4s] OK
  [9s/9s] OK
* checking re-building of vignette PDFs ... NOTE
Error in re-building vignettes:
   ...
Loading required package: car
Loading required package: MASS
Loading required package: nnet
Error: processing vignette 'HE-examples.Rnw' failed with diagnostics:
cannot open file 'fig/plot-plastic1.pdf'
Execution halted

* checking PDF version of manual ... OK

I am unable to determine why the file fig/plot-plastic1.pdf cannot be 
opened. It is in my inst/doc/fig directory & is regenerated
by the .Rnw file. What could cause this?

Second, I have tried manually running 
tools::compactPDF("HE-examples.pdf) on the .pdf file under inst/doc in 
the package,
but no change is made to the .pdf file. I can't see any way to correct this.