[PyMOL] 1 residue missing from my pymol structure

Dear all,
when I open my pdb file in Pymol one residue in each subunit of my
protein dimer is not visible.  I know it's there as I can select it and
it is visible as sticks or lines with all bons present.  I have tried
using a pdb of just the 2 helices in my dimer but the residue still
isn't visible.  I have also checked the pdb and everything seems fine
(the residue can be seen in 'O' and is joined to its neighboring
residues).  I am using version 0.90 in Windows and have used version
0.68 in Unix.
Does anyone have any ideas how I can overcome this?
Thanks,
Claire