the question is a little vague, so Im not sure this is what you are looking for but here's a few possibilities.
Depending on how you have IPSec configured it will not work thru a NAT. IPSec calculates its hash including the header, so when the header is re-writen buy the nat device, the receiving end discards the packet because the hash isnt correct.
Also you are doing PAT. If you have 1 vpn tunnel that works, but consecutive ones screw everythign up that is why. For that the consecutive devices end up stealing the port from the first connection, which then fails. The work around for this is NAT-T.
Can you supply more information about the topology and configurations.
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