The problem you're having is probably because subform2 has not loaded when
you try to call the code; when you do it a second time, it's there.

Rather than use a filter, why not "link" the two subforms using
LinkMasterFields and LinkChildFields properties of the subform2?

Here is how you do this:

Put an invisible textbox (name it txtKitItem) in the main form (form header
section is fine). Set its control source to an expression similar to this:

=[subform1].Form.[ControlNameWithLinkingValue]

where ControlNameWithLinkingValue is the name of the control in subform1
that holds the value for KitItem in subform2.

Then put this in LinkChildFields property of subform2 control:
       KitItem

And put this in LinkMasterFields property of subform2 control:
       txtKitItem

Now the two subforms are linked.