Is it normal for a girl to go in ... as long as she raises her hand, the bra will slip over her chest? It's no use buying a smaller chest-squeezing and relaxing shoulder strap. .....

It's a shoulder strap. Try to use thin straps for shoulder straps, which are more elastic. Don't use broadband. It's too wide and not elastic enough, so it's easy to put the whole bra on. In addition, your bra cup is not big enough, you should buy a bigger and deeper cup, and the sling in the middle should be suitable, not too big.