I've got it!
It was an error on my references.
=Nz([TotalAerosolsSubform].[Form]![AeroValue],0)+Nz([TotalBoltsSubform].[Form]![BoltValue],0)+Nz([TotalNutsSubform].[Form]![NutValue],0)+Nz([TotalWashersSubform].[Form]![WasherValue],0)
Thank you all for your help once again