Try going to http://localhost:631/ ( CUPS printing backend page ) and
check all jobs for the printer. You may have some pending ones that
are stopping the PDF print from being processed. Might be worth
looking at the error log in there too if it shows anything, mine
appears to be blank but then again I haven't had any errors

Is it just one particular PDF that won't print - i.e. will other PDF's
print successfully ? It's possible for a PDF to be protected against
printing and that flag may be set.