The snare/snares sound really good, but I feel there's a lot of high-mids going on. For that, I would make it a tad bassier. The kick needs more low-end, but don't overwhelm the higher frequencies, you actually wanna cleanse of them. I feel like the chords could be more bassier too, imo. Otherwise, you did a fantastic job!
He's saying there's phasing, or muddier frequencies building up in the stereo, which is causing the 'thin' sound arrangement. So I would probably use several layers (one mono, the other stereo,) and that should overall balance your sounds.