Yes, some strains will not have any/many amber trichomes due to their genetics and this is the flaw in the oft quoted theory of "30% amber trichomes" repeated by some people like it is the only way, without ever questioning if it is suitable for every plant and in every situation.
I would be harvesting asap, before even more THC breaks down into CBD and CBN, 2 non-psychoactive cannabinoids.
Since all of your plants pistils have turned brown, peak stage of health has well and truly passed, and she is on her journey towards death, meaning she can no longer produce new "replacement" THC faster than the THC she already has made is breaking down into CBD and CBN and this is all due to her declining metabolism being unable to support new THC synthesis efficiently.
This is why late picked flowers often produce a heavy and sleepy body stone effect, due to the elevated CBD and CBN levels and the diminished amount of THC.
If you prefer a more mind bending THC influenced head high, harvesting any future plants when there are 90-95% brown pistils, will produce that sort of effect.
Hope this helps, Organoman.