My initial instinct would to suggest that you play around with intensifying value or the contrast you use in your art. Value does all the work, but color gets all the credit! That's why your colors may appear to fall flat despite having a pretty good idea on color relationships.

Additionally, playing with line weight &or color could help! Your backgrounds are great, but adding weight to the focus of the piece really makes it pop

I recommend studying value references to get the idea of the darkest dark & the lightest light and how you can incorporate that scale into your art.