Mixing powder paint and glitter can be tricky!!! It took me a long time to get comfortable with it and when I first started I got some flaky ones. It happens to all of us.
Problems mixing range from glitter size to not enough clear in the mix so the glitter flows right and isn't bumpy. One piece of advice I was given to begin was buy clear powder paint by the pound!!! You really don't realize how much you use mixing.
This is a two coat mix - copper penny with a clear coat of irridescent orange glitter which I've posted before
Here are two that have the glitter mixed right in and they took a while to get right
My glitter comes from JoAnnes, Michaels, AC Moore, Hobby Lobby (only get there when I'm back home) and Walmart. I use fine and extra fine, and all of them work. Brands include Creatix, WOW, Nicoles, JoAnnes House brand and Tulip. Several other boards I'm on have sections on paint mix's and the amounts used, including glitter coats.
If you need more help just let me know and I'll try and help you out, but mixing powder paint can get just as addictive as any other tackle making.