Frequent Error In Referring / Linking Textbox In Subform From Text Box Within Main Form

Go to frmInvoice, and you'll see a Net Total box (txtNetTotal) . It's control source is linked to a textbox in the subform fsubInvoiceDetails2 called txtStocktotal. It basically just pulls up all the costs associated with that InvoiceID.

The reference mechanism is as follows: =[fsubInvoiceDetails2].[Form]![txtStockTotal].Now...sometimes this works and sometimes it doesn't! Sometimes i've had to use: =forms!fsubInvoiceDetails2!txtStockTotal.

It seems to be very temperamental at times and i'm not fully confident if this can be explained.By way of note, I use express builder normally to input these statements: I go to Forms > ALL FORMS > fsubInvoiceDetails2 > txtstocktotal.