well there could be a million and one reasons. but first i thought ts-21 was a semi-metallic?!? anyhow, well for one thing different pigments react different ways so it maybe flatter than the yellow. also, paints react to different base coat colors. for metallics some times i use a black base coat, sometimes a silver base coat. for others i usually spray a coat of white first. also the humity and temperature have a big effect on curing so did you paint these two in different seasons? also aplication is a constant variable. you never spray exactly the same every time. also you should at least do the last coat of paint a little on the wet side but this is risky with both laquer and spray cans. you might be able to polish it up to desired gloss. if not just give it a quality high gloss clear coat and polish it. this always helps. try to experiment with base coat colors any time you use a new color! this helps in the long run.