Where you have this Dlookup? in the main form ? in the suform?
=DLookUp("[PO No]","POtoLOTtbl","[Lot No] =" & [Forms]![MainForm]![SampleChemistry].Form![Lot No])
or if your Lot No is txt field
=DLookUp("[PO No]","POtoLOTtbl","[Lot No] =""" & [Forms]![MainForm]![SampleChemistry].Form![Lot No]...