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According to my research, princess flower trees do not respond well to transplanting and may lose leaves or stop flowering until they are established. The best thing you can do at this juncture is to follow the care plan for a princess flower, which is found here:

https://www.gardeningknowhow.com/ornamental/shrubs/princess-flower-bush/growing-princess-flower.htm

Hello,

Thank you for sending us your gardening question. Pruning is part of regular tibouchina care. Remove flowers after they fade, to encourage the shrub to continue blooming rather than using its energy to produce seed. Use a pair of pruning shears to cut the flowers off below the spent blooms, just above a set of leaves.
Prune lightly in the late winter or early spring when the leaf buds begin to swell. Prune to improve the plant's shape and reduce its size. Select tall and misshapen branches and cut back to a parent stem, which is the larger branch from which it originates. Also, thin out crowded areas, especially in the center of the plant. A good rule of thumb to follow is never prune or thin out more than one-third of the shrub each year. Cut damaged and diseased branches back to the parent stem or just above a leaf bud. Do this type of pruning any time of the year, as necessary.

Here is an article you might find helpful:

https://www.gardeningknowhow.com/ornamental/shrubs/princess-flower-bush/growing-princess-flower.htm

Please let us know if you have any other gardening questions and happy gardening!

Thanks
Gardening Know How

Make sure you are adequately watering your plant.
Fertilize your plant with an acid based fertilizer, Azalea and Rhododendron formulas will work.
Follow the application directions on the packaging.
If plant grow still seems poor you should have your soil tested.
If it is extremely alkaline (which I suspect), you can work in pine needles or redwood compost to make the soil more acidic.

Here is an article with more information.

https://www.gardeningknowhow.com/ornamental/shrubs/princess-flower-bush/growing-princess-flower.htm

Make sure the soil stays moist, with short periods of dry in between. This seems like a case of either under or overwatering. Also, to keep the pH balanced, it is a good idea to add dolomitic lime to the soil. This supplements calcium, magnesium, and acts as a buffer for the pH of the soil.

Yes, unfortunately. These can only survive down to zone 9. & will be too cold, and will kill it off during the winter. You will have to grow this in container and bring it indoors in the winter. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/princess-flower-bush/growing-princess-flower.htm

If there is a drought, simply provide more water. These are quite drought tolerant, though. I'm not familiar with the climate of your country, except that it is the opposite of ours at any given time, so I suspect that you are entering or are in your warm season. If it was cold during your winter, then it could still be recovering from the cool weather. They also benefit from yearly pruning. If this hasn't been done, then this could help you here, too.

This article will offer more information on the proper care of the plant: https://www.gardeningknowhow.com/ornamental/shrubs/princess-flower-bush/growing-princess-flower.htm