receiving delivery issue

Hello,

I'm using the following configuration on my kannel on 3 gateways, each
gateway contains 2 sessions, one for sending and the other for receiving,
the below is for receiving delivery status.

I have no issue with all of them till 3 months before one of them not able
to pull the status, at the same time the same gateways is connected with
more than 6000 clients with no issue at all.

The provider asked me to change the register_dlr=1 , any help plz?

interface-version = 34
host = xx.xx.xx.xx
port = 0
receive-port = 8899
smsc-username = user
smsc-password = pass
system-type = VMA
source-addr-ton = 5
source-addr-npi = 1
dest-addr-ton = 0
dest-addr-npi = 0
keepalive = 600
reconnect-delay = 3
enquire-link-interval = 30
esm-class = 0
msg-id-type = 0x01