Are the two subnets connected to different interfaces on the ASA?
If they are and the interfaces are the same security level then you need to add a command "same-security-traffic permit inter-interface". It's under the Configuration > Device Setup > Interfaces section in ASDM.
If the interfaces are different security levels you need to use an access-list.
http://www.ciscopress.com/articles/article.asp?p=1924778&seqNum=5