Summary field not working--suspect key and reference field issue

I'd recommend you build a Months table, and set up a few formula reference fields.  Then all your summary issues will go away, and you can have accurate monthly forecasts for all the projects you need.

Another option is build a summary report on the "forecast" table with the values you want to keep track of, and then create a report link field on the "projects" table and use the summary report.  That should give you a nice list of all the forecast months on the project form.

Just some other ideas...