A quick one - setting filter based on calculated control

Dear experts -
I am trying to set a filter on a subform based on a control on that subform. The control in question is a checkbox, which is based on a formula (a complex one).

I am attempting to build a string and then setting the filter on the subform to that checkbox being true. I look at the subform BEFORE applying the filter and the checkbox is showing up as correctly checked.

I then apply the filter, using the code below, and no records appear.

    If Me!chkNeed2ndApp = True Then strfilter = strfilter & " and [Forms]![Project].sfPartsLines.Form.chkNeed2ndApp = TRUE"

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(Me!chkNeed2ndApp is the control in the parent form that invokes the filtering sub)

Any idea what's wrong with my syntax/approach?

Many thanks.