First, I would check to make sure that it's getting as much sun as possible. Bottlebrush needs as close to at least 6 hours of direct sun a day as can be managed.
Second, I would make sure that it is getting pruned at the right time: right after bloom. The buds for next year's bloom form on the tips of the growth that forms after bloom. Too much frost can kill the flower buds, too.
Third, I would feed them lightly with a low phosphate food. Like many Australian plants, they are adapted to low phosphate soils, and a little nitrogen for growth, and a little potash to stimulate flowers is all they need.

If its in a pot, is it possible it needs a bigger one?

I have a Bottlebrush which has never flowered yet, have had it about three years in a pot. Am planning on moving it to a bigger pot but there are signs of 'something' coming, so I'm on hold to see first. I will move it into the sunshine too, thank you!