The band of your bra seems to dig into your skin or is stretched to the limit.
The band size may be too tight . You could try a bigger band size . For example going from a 34 to a 36.
If you go up in the band size you may have to go down in the cup size. For example 34C = 36B

Shoulder straps are digging in and leaving marks:
You may need to loosen the straps. If this doesn’t work it maybe that the band is too big and the straps are taking the weight of the breasts. The band gives approximately 75-80% of the bra’s support. You may try a size less in the band. For example going from a 38 to 36.

If your bra rides up at the back:
The band size maybe too big. Try lifting your arms up to see if the band is tight enough. If it rides up you may need to tighten the hooks at the back or go down a band size. IE/ going from a 36 to a 34.
If you go down in the band size you may need to go up in the cup size.  For example from a 36C to a 34D

Your bra straps slip off your shoulders:
You may try a bra with wider straps for more support  or adjust the straps to fit tighter.

The bra cups have excessive fabric:
Your cup size may be too big or all your breast tissue may not be placed properly in the cup.
If this doesn’t work you may have to go down a cup size. For example going from a C cup to a B cup.

Your breast tissue is bulging at the sides or over the top of the bra:
The cup size may be too small. You might try a bigger cup size. For example going from a C cup to D cup

The breast form/prosthesis is pulling to one side of your chest.
The band size may be too big. You may need to try a smaller band size. Another issue may be the breast form is too light.