When I have this type of issue, I usually find that there is a referenced document somewhere in the chain that is not open in memory.
You have stated that you have the documents open, but double check that all are open simultaneously. Check the Find References list of each file and ensure that all of those files are open in memory. Rebuild each file from the bottom reference up the chain. If you have configurations, activate each configuration and perform a rebuild. Do this for every file that has configurations.