keepalived + nginx Load balancing, cannot access VIP - nginx

Question

server A(master): nginx 1.9.9 + keepalived 1.2.19, IP: 9.110.95.90
server B(backup): nginx 1.9.9 + keepalived 1.2.19, IP: 9.110.95.91
VIP(virtual IP): 9.110.95.95
nginx bind on all interfaces.
iptables configuration:
iptables -I INPUT -i eth1 -d 224.0.0.0/8 -p vrrp -j ACCEPT
iptables -I OUTPUT -o eth1 -d 224.0.0.0/8 -p vrrp -j ACCEPT
Keepalived configuration:
server A
global_defs {
router_id LVS_MASTER
}
vrrp_script chk_nginx {
script "killall -0 nginx"
interval 2
}
vrrp_instance VI_1 {
state MASTER
interface eth1
virtual_router_id 51
priority 150
advert_int 1
authentication {
auth_type PASS
auth_pass my_pass
}
virtual_ipaddress {
9.110.95.95
}
track_script {
chk_nginx
}
}
server B
global_defs {
router_id LVS_BACKUP
}
vrrp_script chk_nginx {
script "killall -0 nginx"
interval 2
}
vrrp_instance VI_1 {
state BACKUP
interface eth1
virtual_router_id 51
priority 100
advert_int 1
authentication {
auth_type PASS
auth_pass my_pass
}
virtual_ipaddress {
9.110.95.95
}
track_script {
chk_nginx
}
}
It works fine at beginning when accessing the VIP (9.110.95.95), and it also failover to backup node successfully, if stopped keepalived on master node.
Everything seems to be fine, BUT the client failed to access the VIP after some time later, probably several minutes. and if access the server A/B IP(9.110.95.90, 9.110.95.91) it works.
Not sure what's the issue, any ideas?
Thanks