i'm not the best artist around here, but i'm good enough to give some advice on this. i think on the pokemon above your work needs to be on the mouth. but on the others, i think with the heart the main issue is actually your line art. the shading isn't that bad but the shape isn't a heart.
if you don't know what a love heart looks like ask any girl you can find i'm sure they would know, or a google search...
i think as an artist your palette skills are really good but your main issue seems to be your skills in drawing what you want the shape is all over the place. and i think it helps destroy the look alot. from there i would recommend finding a tutorial on shading. it isn't shading in general i'm refering to when i say that i mean so you can find a way of shading but getting rid of that big black outline from your peice.
i would fix up the heart for you just to show you what i mean but right now i'm trying to get something soughted out in my own topic.