An occasional soft shelled egg is not necessarily a calcium issue. Some hens have a genetic problem that does not allow them to process the calcium properly. It does not matter how much calcium you pour down them. They don't proces it the right way.
The hen's shell gland produces a certain amount of shell material a day. It can vary, but normally this is enough material to coat one egg. Sometimes hens release an extra yolk, two or even on rare occasions three, instead of the normal one a day. If these start through the hen's internal egg laying factory at the same time, you might get a double yolked egg. If there is a difference in when they start through the system, you might get a second egg in the same day. Often when this happens, the shell gland does not have enough shell material prepared for the second egg so that egg is soft shelled.
If most of your egg shells are hard but you occasionally get a soft shelled egg, it is probably not a problem with how much calcium they eat. It is probably a problem with an individual hen.