Which is in control of your dancing- your pointe shoes or your foot muscles?
Of course you want to get exactly the right fit. So say your ballet store only sold Freeds, for example. Lengths and widths are commonly available. Freeds are hard shoes. They are wonderful for the higher arch, giving lots of support. They come in low and high vamps, good for short or long toes, so really they could be okay for everyone.
If you have a low arch and less flexible ankle joints, you need to break in your Freeds more. Same with Capezio's. Gambas are lighter shoes, go easy on them. Probably the first couple of classes will break them in fine.
If your core muscles are weak, if you are still hyperextending your knees, if you haven't strengthened the sole of the foot muscles, then your pointe shoes will be in control.
If they don't fit perfectly (no shoe does, unless it's made for you), if you are wide at the metatarsal area, or forefront of the foot, and narrow at the heel, or vice versa, you will always have a little situation.
Fitting Pointe Shoes
Fit the pointe shoes for the larger foot. (You may do the opposite with leather soft shoes, because they will stretch to fit.) You can pad the shoe for the smaller foot a little more, rather than crunch the bigger foot and get bad blisters or an injury.
If you need wide pointe shoes for the metatarsal area, leaving the choice of pointe shoes too wide at the heels, then use a drug store brand heel grip or get a big bag of makeup wedgies and cut them to the exact size you need and glue into your shoes.
I'm not going to go through every nuance of a shoe fit. The Perfect Pointe Book tells you all those details.
Your foot muscles have to be strong enough to control the shoe.
Your posture has to be correct so that you can be on balance.
If you get a pair of shoes with a slight deviation or a spot on the top of the box that presses into your foot, work on it with your hands, or use extra padding. Whether you stick a little foam, corn pad, or adhesive tape on your foot or on the shoe, it doesn't matter. Whatever works.
After two to three pairs of the pointe shoes that you can get still don't work out, you may have to try ordering a different brand on line. You are not going to waste a pair of shoes.
You will learn how to work on the feel of your pointe shoes to suit yourself.
You have to get your muscles in charge.
That's exercising every day, not doing bunches of releves or retires releves, but doing the basic sole of the foot exercises.
In other words, if you have a problem, look at your foot strength first, and your shoes second. Pick a pair a little too large rather than a little too short if you are in between. Too short, and too narrow, can lead to pain and injury.
The Perfect Pointe Book provides you with all the details for developing foot strength (and correct posture too!).