Hi Lisa, we really need to know what part of the planet you are living in, please. City and state at least, or country? Your zone only tells us how cold, on average, it gets in winter. Could be anywhere from the Pacific Northwest to northern Florida. Humidity, strength of the sun, altitude, air pollution and other factors all count towards plant success.
I'm in central Florida and Tibouchina are garden plants here. They like morning sun, not all day. Also rich soil, consistent fertilizer and water. My mother grew it in Vancouver BC for years as a house plant that went outdoors in summer. Her patio also got full, morning sun until about 2pm.
The most likely cause for yours not to bloom (I assume that's the problem?) is not getting enough sun. What's your fertilizer regime for it?
