It’s frustrating, I know. The critters like your bulbs as much as you do. Here are some ideas to keep bulb robbers at bay:
Voles: For starters, volves are my nemeses. They are my worse garden enemy. Have you tried spraying your bulbs with Ropel, Deer Off, Plantskydd, or another bad-tasting concoction? Spray a ready-made product or a home-made, garlic based mix, allow to dry before planting, then surround the bulbs with a sharp crushed gravel or product like PermaTill.
Deer: I feel your pain, although I’m not plagued with deer. I do have many clients with deer problems, I have a few tricks up my sleeves. Try folding a piece of poultry wire around the edge of your garden. Some gardening friends have had success with putting human hair and soap in the area to deter deer from investigating further. Pepper stray can help once the bulb is up, just be sure to reapply after it rains.
Squirrels and chipmunks: Poultry wire will also work well here too. Place wire over the buried bulb area and cover to hide with mulch or leaves.