Mine are always late to flower as well; I just noticed some bud formation happening the other day. It probably has to do with day length. Since they come from areas closer to the equator, tibouchinas may bloom only when the days shorten to the length of those in the tropics, which is less than ours in May, June, and July, but longer than in the winter. That's my theory, anyway.

That is what I thought as well, but these years I have one plant that is willing to flower and four or five that do not make any flower buds at all.

This is the first time I ever grew Princess flower. I bought mine in May and it has been blooming profusing all summer. Rather than day length, I wonder if they might be sensitive to excessively warm day/night temps. (Summer was normally warm here--nothing extreme). I also tend to grow mine in a smallish-sized container--some plants seem to prefer being slightly pot-bound. Good luck!