My understanding is that these are desert plants so it could be that they are getting too much rain or water.
Doesn't Houston have high humidity? I'm in Northern Calif. in the most northern part of the valley. Very dry during the summer and low humidity. Here is some info that might help you.

When growing Mexican bird of paradise, you should keep in mind its overall size, which can reach up to fifteen feet tall with a similar spread. This plant is considered drought tolerant, thriving in well-draining soil and plenty of sun. While it can take some shade, its blooms will not be as profuse in these areas.

Until it becomes well established in the landscape, you’ll need to keep the plant watered weekly and it may require fertilization while in bloom.

Once established, Mexican bird of paradise requires little care, other than the occasional pruning to keep it manageable and neat. This is often performed in winter (when it dies down naturally) and is usually pruned a third back or to the ground. Those grown in pots can be overwintered indoors and cut back as needed.

Hope this helps.