Hello Mark,
Can you post the configuration of your interfaces? It would be easier to advice.
If you are using public IPs, sanitize the config before posting.
I don't have a lot of experience with AWS, but if you are using the same subnet there, I don't think you can assign it's IPs to multiple ASA interfaces. If possible you should divide the block of IPs, make a static route from AWS to ASA (to the divided subnet), and then apply IPs from different subnets (that you routed from AWS to ASA) to ASA interfaces.
I hope it's how it works with AWS. :)
Thanks.