I'm not sure your code Below is correct?
[Forms]![My_Main_Form].[Form]![My_Subform].[Form]!My_Subform_Control]
The Bang Here:- ]!My_Subform_Control] donsen’t look right?
Also, I don't think it truly represents the situation.
Microsoft creates a special Control
My reason for thinking this is when you add a subform to main form Microsoft creates a special control, an intermediary control between your main form and subform. The problem is, Access gives this control the same name as the form (the subform) that is housed within it!
It's just a normal Form!
When you see a main form with another form on it, it is natural to term it a sub-form. But really there's nothing special about the form itself, except for the fact that you can see it on top of your main form. But otherwise, it's just a normal form!
A hidden Control you cannot see
There's a hidden control you can't see, around the form. It looks like a frame, a border, which you can click on and if you click on it just right (a difficult skill to master) but if you manage to select it you will see that it is called a subform/subreport control. You will also notice it has the same name as the form that resides within it.
Microsoft - not at all helpful!
This "helpfulness" provided by Microsoft Access, is as you can see, not at all helpful! It is so misleading and so confusing there's no wonder people have trouble with subforms and subreports!
A Subform Window
I immediately rename this control to something which explains exactly what it is, in terms of what you are seeing. To me, it is a window which contains the subform. I call it a subform window and I give it the naming convention prefix subFrmWinFormName where "FormName" is the name of the form housed within it.
Now with this information, rewrite your code in this manner:-
[Forms]![My_Main_Form]![subFrmWinMySubForm].[Form]![My_Subform_Control]
For More, See My Blog on Subforms:-
http://www.niftyaccess.com/sub-forms/
And my YouTube Playlist HERE:-
https://www.youtube.com/playlist?list=PLhf4YcS5AjdojT2lrV5j1VqDfC1h3Yk1A
Basically a compilation of any of my videos that mention a Subform
Bangs and Dots
I'm never sure if I get them right however I use this Simple Rule:- If it's a name I've given it, use a bang if it's an access name then use a Dot. However I'm never %100 sure if that works throughout MS Access in all situations. For more information read this:-
Understanding Form Structures and Coding
http://c85.cemi.rssi.ru/access/books/A97ExSol/index12.htm