Would be able to post your workbook(mock data is fine) reflecting your data structure? Thank you
Your are forecasting probable on month level. Because the month level is less aggregated you should bring this in first. Because you are bringing this measure in first, this datasource will be declared as the "primary source". If you bring in the actual on day-level you will be able to "Datetrunc" ore "Datepart" orderdate to order month at the same level as you are forecasting.
If you would bring in first actual, budget this will be the primary datasource and with a lower aggregated level day. Coming in with Forecast on month level can't be allocated on the individual days.
Hope it helps...