I've just encountered a similar issue. Went through all the SCCM configuration. Set up the policy. Deployed it. Everything looked good, but encryption just wouldn't start. I ended up revisiting some old MBAM documentation, which required the following registry DWORD to be created:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MBAM
Create DWORD called NoStartupDelay
Set the value to 1
Restart the SMS Client service, and the bitlocker service. Encryption immediately started.