Sometimes the optilon or YKK needs a tiny bit of help ...  If you closely
examine the slider to see how it closes the coils you may find that there is
too much clearance between the top and bottom of the slider on the end where
the coils exit when zipping closed.  Verrry gently using a small pair of
needle nose pliers close this gap a teensy bit.  Then try it again.  Close
it very slightly more and it may do.  Nothing ventured ...

pensioner