Hi Grant,
What is your topology like? Can you paint a much precise picture of what you are saying please. Are you the one in control of the ASA?
Clearly it's a routing issue but we need to know how you are sending the packet along to your remote site...i mean what are the hops you are paying through to get to your remote site! Very well the ASA knows about the subnet but whats the routing btw you ASA and the next hop, like you switch svi or something.
Giving a good vision of what your network looks like we might be about so resolve your problem.
Cheers
Teddy