If you fail a DEILD, the first thing you could think about was whether you were focusing too much on your real body. It's best to have as blank of a mental slate as possible, and especially when it comes to putting attention on your physical body. Sometimes if you wake up, you might be thinking, "don't move, don't move..." and focus too much on actually resisting the initial urge to move. This could cause a failed DEILD attempt.

The way I see it is that if I mentally wake up too much, and put all my attention towards the real world, my mind will know I'm awake and avoid sending me back into a dream.

This next one may be less of a possibility, but you might not have been in the right time of your sleep cycle. That means dreams probably won't form regardless of whether you stay still or not.

General tips I can suggest are to keep your breathing level steady - avoid holding your breath or breathing in a funny pattern, and imagine the sensations of a dream. This can mean tactile (touch) sensations like holding a bowling ball, sight, like visualizing you're in a bowling alley... If you imagine the dream you just came from, it might help the DEILD go along quicker.