IPSEC tunnel status up but no traffic, Have to restart tunnel for traffic to come back up

[zeki893]

Hi I'm running IPsec tunnel between two fortigates. 

The tunnel status shows up, but once in a while I can't ping the other end until After I restart the tunnel.

 I have autokey-keep alive and auto-negotiate checked on both sides.

Anybody know what's up with this?

[gschmitt]

Do both devices have a static IP (or DDNS Hostname) or is one of them a dialup client?

[zeki893]

It is static, not dialup