All the Port's untagged to the Vlan are Blocking status

[Dilashan_Vajira72]

Hi
I am new for the DATA.
any one can help me?
Network topology
1. OS9900 X 2 core switch (one core is switch off)
2. OS6560 Access switches stacking with maximum 4 No of slave switches among the 8 No of flows.
3. All the Access switches are connecting through fiber with the core switches.
4. 10 No of Vlan's configured and ports are untagged to the Vlan's.

Issue description
currently Vlan 20 and 30 using for the CCTV and nurse call devices.
all the port's in the Vlan's are Blocking status.
once the STP Disable it changing to the Forwarding status and working.

what is the reason to blocking status?
if STP disable can it be effect to any loop back to whole network?

Thanks