As has been mentioned, there should be nothing that would cause the bcd436 to not receive calls from only one radio on a system. Now, there are a lot of caveats to this statement. Look for differences between the bcd436 and sds100. I suspect that there is something incorrect in the programming or configuration of the bcd436hp. You should be listening to the output frequencies of the system.
Here are some of the questions I would ask:
1) Is the system properly programmed on the bcd436?
How did you program each radio? Did you enter the system manually? Or did you import from the RR database using Sentinel, Proscan, or some other software? What have you done to confirm that the system is programmed properly on both the 436 and 100?
What have you done to guarantee that the bcd436 (which is not receiving the transmissions) and the sds100 (which is receiving the transmissions) are programmed identically? ( I suggest keeping your environment as simple as possible. Create a Favorite List which contains only this system. Disable all other Favorite Lists. Disable close call. Disable all priority modes. )
2) How have you determined that you are not missing transmissions on the sds100? How have you determined that you are not missing other transmissions on the bcd436 from other users as well? (Incorrect observations or assumptions here can lead you down the wrong path)
3) What have you done to ensure that your observations and conclusions are not tainted by defense caused by transmitting in close proximity to the receivers?
Good luck.