A tip I would give regarding drawing clothing is to make sure it has the right shape language. I mean, you could just shade, but it probably won't turn out looking right unless it has a nice silhouette. If you look at how I drew the clothing, there's not many finer details other than minor folds ;P
Of course this plays into the stylistic aspect of how I chose to draw it, but yk.
Another factor that could also be the reasoning why your clothing looks a bit flat is because you might be forgetting the thickness of the fabrics ^^
Thicker fabric means larger wrinkles, but less in quantity. Thinner fabric tends to have a lot more folds being held at tension points.

These are just some basic tips to keep in mind and I could've explained them better, but they still hold up some credibility I think-

sorry for yapping a bit!! ^^"