SERIAL is UP, but no routes to destn

[msasif]

HI, I have strange problem with one of my customer router. The users in the branch will not be able to connect to the HO for sometimes and when I checked the serial it shows (serial is up and line protocol is up) but when I gave(show ip route) I could not see the branch n/w subnet route, I could not be able to ping the ethernet ip address of the branch from HO. When I gave shutdown and no shutdown at the HO serial, I can see the branch subnet and the users can access the server. Iam using RIP. I tried changing the keepalive to 5sec but no use.   Is it h/w problem, but Iam getting this very rarely(twice, sometime in a week).   thanks   imran