True, they need potash and yearly feeding with fish emulsion and/or rotted manure. But they really need sun, and lots of it. I located my 3 trees in my sunniest location and they're doing well. I wonder if the pear trees are shading the peach as well as late sun arriving.
Excess water doesn't help much either as it bloats the fruit and causes it to rot faster. If possible, figure some way to lead away excess rain/water buildup around the peach tree, perhaps a shallow trench encircling that tree with a lead off to another area.