Q: I'm sinking into my pointe shoes. I've tried different shoes and nothing is working. I used to have high arches, but they seem to be less flexible these days. Is this why I'm sinking?
A: I actually believe pronation could be the cause of your sinking. Pronation happens when dancers turn out their feet more than their hips. Ultimately, this weakens the arch muscles, and that's why your feet cramp when you work them. The fact that you've noticed your high arches have gotten lower is a good indicator of pronation.
I'd encourage you to practice pointing your feet without socks or shoes on, so you can observe your toes. Sit on a chair, straighten one leg and slowly extend the ankle, then stretch the toes away keeping them long and separate from each other. You may not be able to get to your regular point—that's OK. You need to first break the habit of crunching the toes under as you point. Don't be surprised if your arch muscles cramp while doing this. That just means you've found the muscles you need to strengthen. Simply reach down and gently massage your foot until the cramping is gone.
When standing, the golden rule is to keep your weight evenly on the three points of the feet: the pads of the big toe, the little toe and the heel. Try doing this barefoot, then in soft slippers and finally in pointe shoes.
It takes time to break a pronation habit, but don't give up. Have your teacher check how you're using your turnout to make sure the pronation isn't coming from working incorrectly.