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It doesn't sound good for recovery. That's a long time after transplanting for it not to respond with new growth.
To assess for viability, start at the top and bend some branch tips. Are they flexible or do they appear brittle and break off? When you get down to larger stems, take sharp knife and slice off a thin layer of bark. Is there a green growing layer beneath the outer bark or is it all dry and brown? If the twigs and branches are all brittle and with no green growing layer, then it is probably dead.
It may have lost too much of the absorbing root system with the transplanting and could not take up enough water and went into transplant shock.

Your plant is suffering from transplant shock, which is sometimes due to poor handling, breaking of the soil rootball, root loss if it was dug from a previous location, or delaying watering and allowing the root ball to dry before planting. Or sometimes it just happens in spite of best handling and watering.
With sandy soil, your challenge will be to keep it watered until it can recover and start to put out new roots and new green growth. Don't write it off as a loss yet.
I like a 'soil moisture meter' to take the guess work out of when to water. Example - https://www.domyown.com/luster-leaf-rapitest-mini-moisture-tester-1810-p-7732.html

I don't know your resources there, but if you can shoot down to a garden center and get some "Superthrive", I swear by it to help avoid transplant shock an aid recovery. https://www.domyown.com/superthrive-the-original-vitamin-solution-enhanced-with-kelp-p-9474.html

Jacobinia pauciflora 'Firefly', Justicia rizzinii 'Firefly'.
DESCRIPTION: Dense growing shrub with loads of fiery tubular flowers shaded from red to yellow late winter and spring.
PLANTING: Plant in a partially shaded-full sun position in the garden, if you live in a cooler climate, plant in a pot and enjoy on a well lit patio.
CARE: Prune after flowering to encourage dense growth habit and continued flowering. Mulch and apply liquid fertiliser each spring. Protect from drought and frost. Give plenty of water during dry periods.

Here is this websites article on pruning bottlebrush. https://www.gardeningknowhow.com/ornamental/shrubs/bottlebrush/pruning-bottlebrush-plants.htm
However, it may be easier to just start over with a new plant. A radical approach is rejuvenation pruning - cutting back an entire plant to 1-2 ft tall. This works for many shrubs but Callistemon's response is "iffy". In other words, it may die instead of sending of new growth. If you decide to selectively prune it back over several years, don't prune out more than 30% of the leaves in any one year.

If you have the common bottle brush, Callistemon citrinus, it is hermaphrodite (male and female flower parts), so any plant of this species should bloom. The answer is most likely an issue with growing conditions or management. Do you perhaps shear the plant and cut off all the flower buds in the process? Have you built soil fertility with compost, mineral amendments and mulch? Do you fertilize or water the non-blooming plant differently than the other? Is there pavement, lawn area that gets daily watering excess, or other compromise in growing conditions?

You haven't provided a photo or much information, but this article has some general information on some factors to consider. And an Internet search showed me several other articles on the subject:

https://www.gardeningknowhow.com/ornamental/shrubs/bottlebrush/bottlebrush-wont-bloom.htm

https://www.google.com/search?newwindow=1&safe=active&source=hp&ei=VpwyXJ__OIfyvgSIxZjgAg&q=+bottle+brush+not+blooming&btnK=Google+Search&oq=+bottle+brush+not+blooming&gs_l=psy-ab.3..0.7212.18868..25148...3.0..0.174.2814.21j8......0....1..gws-wiz.....0..0i131j0i10j0i22i30j0i22i10i30.wGbzrkdHEs4

How warm are you keeping the greenhouse? These dwarf trees are zoned 8-11 so if it is struggling you may need to warm it up.

Grow in average, lightly acidic, well-drained soil; avoid alkaline soils. Water deeply, regularly during first growing season to establish an extensive root system; reduce frequency once established. Apply fertilizer in early spring. Prune to shape after flowering in summer.