[R] using postscript() for multiple individual files

[Ingo Wardinski]

Hello,
I try to plot some results of an analysis to multiple individual 
postscript files, like:
   postscript("A1_zonflow.ps",onefile=F)
   plot(s, type = "vectors", idx = 1:12,main="")
   dev.off()
#
   postscript("vectors_paired_zonflow.ps",onefile=F)
   plot(s, type = "paired", idx = 1:12,plot.contrib = FALSE,main="")
   dev.off()
#
   postscript(file="wcor_zonflow.ps",onefile=F);
   plot(wcor(s,groups=1:50),scales=list(at=c(10,20,30,40,50)),main="")

I expect to obtain three individual files, showing the actual graphics. 
The three files are generated, but the first two appear to be empty, 
only the third file shows the expected graphic.
How can I create three individual (not-empty) postscript files?
I run R version 3.1.1 (2014-07-10) -- "Sock it to Me" on linux

TIA
ingo