There are to find a lot of very interesting publications about Strobilanthes kunthianus, but I could`not find one dealing with flowering hormones or other flower evocation.
This article, "Mass flowering and pollinators of Strobilanthes consanguinea in the Western Ghats, South India" says: "Mass flowering plants in general and Strobilanthes species in particular, may end up with little amount or no seed set in bloom when environmental parameters are not favourable and pollinators are not available."

And it also mentions: "Gamble reported that the mass blooming Strobilanthes species grow so densely that the seedlings of the forest trees could hardly survive under them.

So it could be that the oldest of your plants starts to bloom ( at the first after 6 years ) as soon as the environmental conditions are best for theirs seeds and the number of pollinators is high enough, and it dies after seed dispersal.
The others "wait" till conditions are optimal again and the second oldest or healthiest has a turn.

If you are interested in:

a publication ( abstact ) about Reproductive strategies of Strobilanthes kunthianus

a lot of info about your plant
http://savekurinji.com/

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