hollow pickles when preparing

i have a recipe that has been handed down from my mother. the first five days, i daily wash cukes, cover with boiling water, submerge and set daily. on the fifth day i prepare the brine and cut the cukes in 3/4" slices, when i cut them, they are hollow, the whole seed area has become hollowed out. i picked both very young and slightly larger cukes, i poured boiling water over them less than an hour after picking, they are in a crock for the duration of their submergence, and i use a stainless steel pot to boil my water, i have well water, why are they hollowing out???