I would never keep a cow of any age that calved in February, been running with a bull since May and is open. That is 6 months running with a bull and open??
If you keep leaving the bull in with your cows, or keep making excuses for your cows and keep trying to get them bred, you will NEVER have a "calving season".
Sounds like you have a good mineral program. How about your health program? What vaccines do you use?
Right now, you say you have calves due in December & January - others Feb & March - and one that is 2 months pregnant, so maybe May? Pull your bull. You will be having calves every month of the year.
It doesn't really make any difference what BCS they are NOW. What were they when they should have been bred? That is the important question. If they were thin (you did say one was thin) than either your nutrition was too low or SHE doesn't fit your farm management.
I realize you are trying to build yourself a herd, but building with the wrong building blocks can mean everything falls down.