Bad fit rarely announces itself. It shows up as a red mark, a fidget, a habit of hoisting the straps at the bus stop. Each of those small annoyances points at a specific mechanical problem, and most of them are fixable by changing the size rather than abandoning the style. Read the signs in order, because the band explains several of the others.
The band rides up your back
If the back of the band sits higher than the front, it is too loose and the whole garment has started to hang from your shoulders. A band should be roughly level all the way round. Fasten a new bra on the loosest hook and work inwards over its life, not the other way round.
The straps leave grooves
Shoulder grooves are the classic symptom of a band that is not doing its job. Tightening the straps feels like a fix for about an hour and then makes the grooves worse. Go down a band size and up a cup before you buy another strap cushion, and compare the numbers against the individual brand chart, since ranges published by sellers such as https://just-my-size.com/ differ from one style to the next.
The centre panel floats
On an underwired bra the small panel between the cups should rest flat against the sternum. If it hovers, the cups are too small and the wires are being pushed off the ribcage by tissue that has nowhere to go. This is a cup problem, not a wire problem.
The wire sits on tissue rather than behind it
Wires belong in the crease at the root of the breast, following the ribcage. If one lands on soft tissue at the side, you will feel it within an hour, and the pain gets blamed on wires in general. Nine times out of ten the shape or the cup size is wrong.
- Cup gaping at the top: cup too large, or the style is cut for a fuller upper section.
- Cup cutting in at the side or top: cup too small, whatever the label says.
- Band you can pull far from your back: at least one size too big.
- Neckline showing the outline of the cup edge: usually a seam shape issue, not a size issue.
- Constant readjusting through the day: the bra has stopped holding its shape.
You can pull the band away from your back easily
Two fingers under the band at the back should meet firm resistance. If your whole hand fits comfortably, the bra is decorative rather than supportive. This is the single quickest test to do in a changing room or at home.
It was fine last year and is not fine now
Elastane fatigues. A bra worn two or three times a week and washed properly holds its shape for something in the region of six to twelve months, after which the band relaxes and support quietly disappears. The idea that a good bra lasts for years is a comfortable myth that costs people their posture and their patience.
What none of these mean
None of these signs are a verdict on your body. They are a verdict on a garment that was cut to an average and did not meet you. Clothing should fit the body, not the other way round, and a fitting is just information gathering.
Where to go next
Re-measure, order two adjacent sizes, and judge each one against this list rather than against how you expect to look. The fix is usually a band size and a cup letter away, not a wardrobe overhaul.