Is this a Rosa rugosa? The fruits could be dying because of too much or too little water. Hard to say without knowing your location, type of soil and how you care for them. Is the plant still pushing out new flowers? It costs a lot of energy to make a fruit, so make sure you water and feed your plants while they are making flowers and fruits. Plants need different nutrients in the growing leaves-phase than they do in the growing fruit-phase, so be sure you pick the right fertilizer.

edit to add: I have rosehip in my garden too, but I live in Europe and I planted the native Rosa canina. I find native species easier to care for since they are adapted more to local climate and soil.