OpenVPN route keeps disappearing?

My ERX SFP is an OpenVPN server to two other routers running Tomato. Periodically I can no longer ping the LAN addresses of the client routers, but I can still ping their OpenVPN addresses. Restarting OpenVPN fixes things, but I don't know why this is happening.

In the below, 192.168.1.0/24 is the LAN of one of the client routers, and 10.8.0.3 is that router's OpenVPN tunnel address:

C:\Users\david>ping 192.168.1.1

Pinging 192.168.1.1 with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 192.168.1.1:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\david>ping 10.8.0.3

Pinging 10.8.0.3 with 32 bytes of data:
Reply from 10.8.0.3: bytes=32 time=54ms TTL=63
Reply from 10.8.0.3: bytes=32 time=53ms TTL=63
Reply from 10.8.0.3: bytes=32 time=59ms TTL=63
Reply from 10.8.0.3: bytes=32 time=53ms TTL=63

Ping statistics for 10.8.0.3:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 53ms, Maximum = 59ms, Average = 54ms

OpenVPN config:

david@RoutyMcRouterson# show interfaces openvpn vtun0
 description "Home VPN Server"
 encryption aes256
 mode server
 openvpn-option --client-to-client
 openvpn-option "--keepalive 10 60"
 openvpn-option "--comp-lzo adaptive"
 openvpn-option "--verb 1"
 server {
     client parents {
         subnet 192.168.1.0/24
     }
     client rena {
         subnet 192.168.10.0/24
     }
     push-route 192.168.4.0/24
     subnet 10.8.0.0/24
 }
 tls {
     ca-cert-file /config/auth/openvpn/DaveServer/ca.crt
     cert-file /config/auth/openvpn/DaveServer/RoutyMcRouterson.crt
     dh-file /config/auth/openvpn/DaveServer/dh.pem
     key-file /config/auth/openvpn/DaveServer/RoutyMcRouterson.key
 }

david@RoutyMcRouterson# show protocols static
 interface-route 192.168.1.0/24 {
 next-hop-interface vtun0 {
 description Parents
 }
 }
 interface-route 192.168.3.0/24 {
 next-hop-interface vtun1 {
 description Meadows
 }
 }
 interface-route 192.168.10.0/24 {
 next-hop-interface vtun0 {
 description Rena
 }
 }

Can anyone shed some light on this?