The single most common fitting problem we see is a band that is too big, compensated for by straps that are too tight. It is uncomfortable, it does not support anything, and it is usually the result of being sized once, years ago, on a chart that added four inches to the underbust measurement for reasons that stopped making sense when fabrics got elastane in them.
What the band actually does
Most of the support in a bra comes from the band, not the straps. The band is a horizontal anchor; the straps only stop the cup falling forward. When the band is too loose, the straps take over, and shoulders are not designed to carry that load. That is where the grooves come from.
How to tell
Fasten it on the loosest hook and look in a mirror from the side. The band should sit level — the same height at the back as at the front. If it rides up towards your shoulder blades, it is too big. You should be able to get two fingers under it and no more.
Sister sizing
If you go down a band you have to go up a cup to keep the same volume. A 36C and a 34D hold roughly the same amount; the 34D just holds it with the band instead of the straps. This is the single change that fixes most complaints.
One caveat
A firmer band is not the same as a tight one. If the band is leaving a deep red line or you cannot breathe out fully, you have gone one step too far. Firm and level, two fingers underneath, on the loosest hook so you have somewhere to go as it relaxes.