currently have:

\definefontfeature
 [arabic]
 [mode=node,language=dflt,script=arab,
  init=yes,medi=yes,fina=yes,isol=yes,
  liga=yes,dlig=yes,rlig=yes,tlig=yes,
  calt=yes,trep=yes,mark=yes,mkmk=yes,
  kern=yes,curs=yes]

\definefallbackfamily[mainface][serif][Scheherazade][range=0600-077F][features=arabic]

and get output in which the letters are not joined up into words but
printed separately, as in

ثم يتعرف وزن ما يحتاج اليه مع وزن الكفة لموازاة الافق بالعمود
Is there a setting that I have missed?

Alan