I am not sure exactly which situations work and which ones don't, but typically Access needs more information about the tables and how they relate to one another. You can try setting up a relationship between the tables. Or, depending on the data, set up indexes on your tables. Adding unique indexes (No Duplicates).
You can tell if a query is updateable by creating a Select query and then trying to manually change data in a field. You will see in the status whether or not the query is updateable.
Another way I use to update info, when nothing else works (when using Grouping) is to make an intermediate table. Create a Make Table query that puts the results of your query into a table. Then try the Update query using the table instead of the query.