Modules & VBA :: Form OnCurrent Event Not Firing Properly

We use our CRM for entering client orders, which is done through our 'OrderEntry' form. On that form we have a subform that we use to enter order lines - as we have a range of products on offer, an order can be made up of one product or anything up to 100.We have an 'amount' textbox on the subform, that the users enter the cost of each line, and as they add more lines the total cost of the order is calculated using a textbox in the subform Footer, with the ControlSource set to =Sum([amount]).

On the main form, we have a Net textbox, that the user will add in the total cost of the order once they've added all the order lines, and providing the =Sum([amount]) textbox on the subform matches the figure in the Net textbox, a button to Proceed the order and go on to the next step appears.

The intermittent problem I have, is that the OnCurrent event I use to show the Proceed button does not seem to fire on its own - but when I stop the code and step through it, it will show the Proceed button without any problems.Here's the code:

(the Proceed button is referred to a 'Command80')

Code:
Dim OS As String
Dim UT As String
OS = OrderStatus
UT = fGetUserType

[code]....

I've made the part that refers to the Proceed button bold, but thought I'd add in the whole OnCurrent event in case there was anything in there that was blocking it.

I added in the MsgBox code at the bottom to make sure the OnCurrent was firing, and that works fine.I've searched through the rest of the code, and there's nothing else in there that references the Visible property of the button.I've been through the decompile process detailed here, and also been through this similar thread with a fine toothcomb and this still won't work.

I've saved the form out as a textfile and then imported it back in, both through the immediate window, and short of importing everything in to a new DB and starting again