no glissando between 16th notes

[Frédéric Bron]

The following code produces no glissando between 16th notes (2.12.2):
\include "english.ly"
\relative {
        a'2\glissando b
        a4\glissando b
        a8\glissando b
        a16\glissando b
}

Why?
In fact, I am not really interested in the glissando. I want a
straight line above two successive notes to say that the clarinet
player must glide a finger between two keys and I tought I could just
shift a glissando but it does not work.
Any suggestion?

Frédéric