You can try using the Sum() function. A summary field only reflect the sum of the found set of records as you have found out. A calculation with Sum () can sum up all the related child records.
I am having this same issue.
It is easier to pull data from table in a seperate file than it is from a seperate table within the same file!!
I even tried using a script to copy and paste... still not sure what that isn't working? the field just ends up blank, and if I try to enter data it is automatically erased until I delete the script.
I found a solution by creating a random relationship and using the "x" relationship operator.<!-- POLLS --><!-- FILES -->
Pulling data from a table within the same file is identical to pulling data from a table in a different file. It all depends on the relationship between the tables. Filemaker "doesn't care" which file contains a given table.