Hi, sover,
I love dendrobium hybrids like the one you have, and I grow quite a few of them.
I don't see initiated flower spike in your picture, but if it started a spike and then it won't develop any further, then I don't think it's due to any particular cultural error.
Not enough light is probably the most common reason for no flowers, but yours at least started a spike so light is not an issue here.
Humidity is also not an issue really. I grow all mine indoor by the sunny window and the humidity is low. They flower just fine although the flower count is not as bountiful as their potential could reach.
Sounds like you are doing pretty much everything alright.
Maybe it's just the plant not behaving nicely.
When a spike is initiated, it should develop into a full spike with buds on it. I have one nearly dried up dendrobium with no leaves and roots and still initiated two spikes (one aborted in the very early stage of its development and the other grew and is bearing 7 flowers now).
It's most likely the plant. I would say bye bye after one more misbehavior.
What color were the flowers when you first bought it? Some white dendrobiums won't develop even if they are subject to high light. Even some blue colored ones also stays full green without a hint of purple tanning. It's quite complicated I guess due to their breeding background.
Anyways, just keep growing for a bit longer and if it becomes a pattern, then you have the answer!