VPN loss packet

Hi every body!

My toppology:

(Lan1)—> f0/0[Router1]atm0/0/0.1–> VPN atm0/0/0.1[Router2]f0/0—>( Lan2)
I have configured VPN site-to-site from R1 to R2

ping from Lan 1 to f0/0 of R2 ok
Router1 ping Router2 ok
but when Lan 2 ping Lan 1 packets were lost about 60%

Thanks for your view