What is the calculation? Is it a data field with an auto-entered calculation or a field of type calculation?
Most calculations in calculation fields and auto-enter calculatuion fields that have the "do not replace existing value" option selected will update automatically.
Calculations that use the get function must be placed in fields of type calculation and be specified as Unstored by clicking the storage options button and selecting "do not store..."
Auto-enter calculations will not update automatically both when get functions are used and also when they reference fields from a different table or record.