egg bound hens

I'm so sorry for your losses. You did all the right things, but sometimes these hens just can't be helped. For future reference, sometimes administering a high dose of calcium can be helpful to a hen in an emergency egg-bound situation. It wont harden the shell, but it will make the pushing muscles stronger. Lots of people on here use TUMS.

As for how common it is... it's more common in some breeds than others, I think, and it certainly comes up a lot on these forums. Although, we never hear about the hundreds and hundreds of hens who are never egg bound, because nobody needs to post about that!

A big contributing factor can be diet, so make sure all your hens have the perfect anti-binding diet. The main thing is access to plenty of calcium, usually in the form of oyster shell. Calcium deficiency affects both the strength of the egg shells and the strength of the muscles used to push the egg out. They need enough sunlight too to produce vitamin D, which allows the body to process the calcium.
Also, eating too many dark leafy greens (like chard, spinach, etc) will affect how much of the calcium they eat gets absorbed. These should be fed only in small quantities.

The other important element is essential fatty acids, which hens get from eating things like sunflower seeds. These nutrients keep the membranes soft and flexible, meaning they can stretch to accommodate the egg.