All your controls are belong to us!
Fionnuala answered this correctly but skimmers like me would find it easy to miss the point.
You don't refresh the subFORM you refresh the subform CONTROL. In fact, if you check with allforms() the subForm isn't even loaded as far as access is concerned.
On the main form look at the label the subform wizard provided or select the subform by clicking once or on the border around it and look at the "caption" in the "Other" tab in properties. That's the name you use for requerying, not the name of the form that appears in the navigation panel.
In my case I had a subform called frmInvProdSub and I tried for many hours to figure out why Access didn't think it existed. I gave up, deleted the form and re-created it. The very last step is telling it what you want to call the control so I called it frmInvProdSub and finished the wizard. Then I tried and voila, it worked!
When I looked at the form name in the navigation window I realized I'd forgotten to put "Sub" in the name! That's when it clicked. The CONTROL is called frmInvProdSub, not the form and using the control name works.
Of course if both names are identical then you didn't have this problem lol.