The reduction in volume is typical, decomposed not decamped! The process is completely expected and natural, otherwise we would all be up to our necks in it. The value as a nutrient is only valuable because the process has produced soluable salts which a plant can absorb. The fibre content has broken down to humus which can "space out" soil particles, creating voids which will hold moisture ready for roots to absorb.
Repeat the process, you have succeeded.
Perhaps move the composters onto the uphill end of a plot, so that any run-off passes through an area you can plant, rather than away into a watercourse where damage to the natural flora and fauna may result. A simple stack will also breakdown over time, with less work and better utilisation of space. The formation of a heap alongside an older one will permit the migration of worms etc as conditions dictate. Manure containing large populations of brandlings is not yet ready for incorporation, these worms will not become earth worms useful in soil so let them move away, into the new heap , to perform their magic.