What determines LNAV "circling mode"?

I have been trying to figure out what is happening with my Cambridge
LNAV since in several flights "speed-to-fly" bars have never appeared,
even in a long continuous glide straight at the goal indicated by the
connected Cambridge GPS. Since "speed-to-fly" bars only appear in
"cruise" mode possibly my LNAV is somehow always in "circling" mode -
but I cannot find in my very thin LNAV manual (version 4, march
1995) what determines the choice of "cruise" vs "circling" mode so am
hoping someone here has that knowledge so I can at least know how I
should _expect_ it to behave.

(FYI I do not believe I have the G-meter option, though can't say for
sure since I was not the one who purchased the LNAV new.)