I am exporting a file stroke.org <http://stroke.org/> using the bibliography file stroke.bib in the same directory
The citations are not being incorporated into the pdf file

An example from stroke.log: LaTeX Warning: Citation `Drummond:2014aa' on page 2 undefined on input line 71.

The header for stroke.org <http://stroke.org/> is:
#+LATEX_CLASS: article
#+BIBLIOGRAPHY: stroke vancouver option:-d

My emacs is from MacPorts  GNU Emacs 24.5.1 (x86_64-apple-darwin15.4.0, NS apple-appkit-1404.46)
 of 2016-05-01 on teneleven-slave.macports.org <http://teneleven-slave.macports.org/>

My .emacs file contains
(setq org-latex-pdf-process '("latexmk -pdflatex='pdflatex -interaction nonstopmode' -pdf -bibtex -f %f"))

Org-mode version 8.3.4 (8.3.4-93-g0d72c3-elpaplus (I updated it as suggested here http://orgmode.org/elpa.html <http://orgmode.org/elpa.html> but it made no difference)
I’m using TexLive 2016 with fully up to date packages

Here is the preamble from stroke.tex (no mention of the .bib file)
% Created 2016-06-27 Mon 22:50
\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
..
\usepackage{wrapfig}
\author{David Abernethy}
\date{\today}
\title{}
\hypersetup{
pdfauthor={David Abernethy},
pdfcreator={Emacs 24.5.1 (Org mode 8.3.4)},..}
\begin{document}

My set up (Latex header in the .org file and .bib file with the .emacs entry) used to work for this file but I have probably updated emacs since.
Can anyone see what is wrong?  help much appreciated

David