Smart group set for keyword not picking up some content

Hi there - anyone else hit this?  I have smart groups set up to for selected keyword criteria - sometimes for no reason it doesn’t pick a reference up.  I have confirmed the keywords are spelt correctly in the refs kw section and in the smart group’s pick up section.  The SG has others with the same keyword sitting there fine.

Using Endnote X2 bld 3210 on XP

Can anyone shed any light on this?