Form with Subform and combo boxes not populating properly

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Hello all!

I have a main form called frmEstimate with a detail sub form called frmEstimateDetails. The main form contains record info while the sub form contains record details (so 1 main record to many detail rows). I have 3 combo boxes on the sub form cboWorkType, cboWorkDepartment and cboWorkCategory. I'd like to populate cboWorkDepartment based on the selection of cboWorkType and populate cboWorkCategory based on the selections of both cboWorkType and cboWorkDepartment. Once I have selected values in all 3 dropdowns, I'd like to autopopulate a txt box called txtBaseRate with a the corresponding value from the table. So basically I'm generating the "Where" clause in a sql statement. Now when I create this on a standalone form it works great and I'm using "where WorkType = forms!frmEstimateDetails!cboWorkType" however since it's a sub form this logic isn't working and just keeps prompting me. Can anyone please help me with the syntax to get this to work I'm stumped!

I'm on Access 2010/Windows 7

Thanks in advance,
Kim