peaches that never seem to ripen properly
they were picked way to early
When choosing or picking look for more than aroma and colour.
As a grower of stone fruits we don't wait until they are ripe and soft,just ripe.There is only a 3 or 4 day window for peaches and nectarines,perhaps a bit more with very gentle hands and a great cooler.
One of the things to check is the "flower line",the ridge that is pole to pole on apricots,peaches and nectarines.It should be a bit forgiving and firm to a gentle push with your thumb.If it is hard the fruit won't ever come around or ripen.
Stone fruits don't develop additional sugars off the tree, sadly. They will just soften. It's quite the bummer because there's a peach tree behind my townhouse and the fruits never get far past green.
There are some recipes out there that utilize unripe peaches, in the case that you find yourself with duds: http://norecipes.com/blog/2008/05/24/...
I'm sure there are more if you do some Google digging.