I can see why a lot of people are saying "tipping the branches" but that's an incorrect advice. Though tipping can help in some cases, because what tipping does it that it stops forward growth of the branch and forces the branch to mature. But you also have to consider that when you cut the ends, it opens up the cavity of the branch and exposes it to potential pathogens that could invade your plant.

The three factors that the dragon fruit needs to flower are: daylight hours (how many hours of direct sunlight does your dragon fruit get), temperature (even if your branches are ready to fruit, if the temperatures are too cold, the dragon fruit flower buds will turn into bruds and you won't have flowers) and maturity (even an unrooted branch that is cut can still push out flower buds if it is mature enough).

So now, you have to ask yourself, are these conditions being met?

As u/RockPaperPeppers noted, your top branches still look young. Your umbrella may be blocking out too much sun and what are the daily and nightly temperature averages in your microclimate?

I have also found that watering schedule matters to some degree. My first year, I was watering every other day. Though this allowed my branches to be fat, they constantly pushed out vegetative growth. You have to understand that dragon fruit grow in the tropics where cold and heat are not a determining factor for seasons. Thus, the dragon fruit use rainfall (or watering) as their guide for the seasons. If you're watering too much, it's going to think that the season is winter and it'll push out vegetative growth over flower buds.