1- this happens with low quality pastels that do not smudge, blend and cover well and it is very frustrating. here are some links about oil pastel brands:
2- I don't know whether it is common, but all my drawings look childlike until they are almost finished. So i took these horrible phone photos to show you the ugly truth behind my pictures:
first i put down the lightest colors. it looks terrible.
then I SMUDGE IT ALL! (still horrible)
then i do the sane mature thing and cover it in black (dark colors). things look better in black.
SMUDGE EVERYTHING! (except where you want to keep the texture)
the real magic happens when scratching into the dark areas, revealing the first light layer.