Here are some scraps of information I can give you from Sunset's Western Garden Book re. Tibouchina heretomalla:
"Evergreen shrubs; Zones vary by species; Partial shade in hottest climates; Regular water.
"Brazilian natives with showy flowers at branch ends and broadly oval, prominently veined, velvety leaves. Prefer slightly acid, fast-draining soil. Protect from strong winds, hard frosts. Pinch young growth to induce branching; prune out damaged or badly placed branches before new growth begins. If flower buds fail to open, check for geranium (tobacco) budworm.
"T. heteromalla. Zones 16, 17, 21-24; H1, H2. Shrub. To 4-6 ft. tall and wide. Silvery green leaves with silver undersides to 6 in. or longer. Dark purple, 1-1.5-in. flowers in 1.5-ft-long clusters from early fall into winter."
The Sunset climate zone map of "Los Angeles and Inland" shows some cities and towns by name. Fortunately for me, Culver City is one of them; it's on the line between Zones 22 and 24, so T. heteromalla should feel quite at home.
Here are possibilities I pick out from the Sunset language:
--Maybe the "slightly acid, fast-draining soil" isn't to its liking?
--I can't believe that Zone 22 qualifies as a "hottest climate" and therefore requires shade, but maybe in young or newly transplanted specimens, shade cloth is helpful until the plant becomes established....
--Or maybe it's simply settling in. Your neighbor's plants have been there for years; yours, for three months. Is this a possibility?
Hope all is well very soon.
Sunset Zone 17, USDA Zone 9