Melodic Ideas in the Same Register
This is one of the first rules one learns when learning to orchestrate and still I very often see it being done wrong: make space for important melodic lines. You don’t want to have several lines and textures in the same range walk on top of each other.
This again is often a problem of sequencer composing as it is harder to spot gaps or possible conflicts than it is on score sheets. The essential thing is to constantly remind you to not have several melodic lines in the same register conflicting with each other. Make sure to consciously listen to your music for such problems. Overlapping melodic ideas only work when either the texture or the volume is drastically different from each other.