Uncertain behaviour of subprocess in python

Question

snmpset -v 2c -c private <oid> integer 6
try:
    x = subprocess.check_output(command,shell=True)
except Exception:
    msg = "Some error here"
    exit()

I have written a python script of which the above code is a part. Sometimes it is executed properly while sometimes it gives the following error:

Error in packet.
Reason: noCreation (That table does not support row creation or that object can not ever be created)
Failed object: SNMPv2-SMI::<oid>

I am not able to understand why it is behaving in an uncertain manner. Can anyone please help? Thanks!