For some reason, I am having a lot of trouble caring for my 2 dragon trees. I have got about 4 houseplants + plants in the greenhouse and garden, ranging from carnivorous plants to palm trees, and in between. Why am I having so much trouble with my dragon trees?

I use the same soil I use for my other plants (coir), which isn't always the best for all plants, so I only use it on the plants that will benefit from it. However, it does not seem to work for my dragon trees, even though I have added perlite and gravel to aid drainage (the same as my other plants [including cacti, which are also growing very well]). I have also tried moving them around to see whether it was the light conditions, but no change. I have unpotted them at least 4 times since I bought them earlier this year because the soil remained soggy for too long. And, as soon as I bought them I repotted them, as they were very pot-bound, but I didn't go a pot size too big (if anything too small), so that can't be the problem. The humidity is fine and the temperature is fine.

What else could it be?

My first dragon tree

My second dragon tree

Sorry that picture's blurred, you can still see the brown leaf...

Thank you!