DB Combo not picking up changes

(using VB 6 SP6)

I have three DB combos that have change activations on them. effectively you select a system then a magazine, then an issue. On change of system the magazine list changes, on change of magazine the issue list changes - pretty much a standard 3-level hierachy.

The problem I am having is this. The code below shows 3 variables being assigned to the boundtext properties of the combos (which should activate the on_change code). The code below doesnt activate the on change code.

frmPurchaseOrderMaint.dbcSystem.BoundText = sSystem
frmPurchaseOrderMaint.dbcTitle.BoundText = sMagRef
frmPurchaseOrderMaint.dbcIssue.BoundText = sIssueRef

if I change this to have literals like this:

frmPurchaseOrderMaint.dbcSystem.BoundText = "NS"
frmPurchaseOrderMaint.dbcTitle.BoundText = "1001"
frmPurchaseOrderMaint.dbcIssue.BoundText = "20070001"

the above code DOES activate the on change code.

if I put a break point just after the first example and look at the contents of the variables sSystem, sMagRef and sIssueRef they have valid values, but if I mouseover the combo fields, they all show "" as their value (this explains why the on change code isnt activated, but why does an assignment using variables not work, whereas assigning literals does. it doesnt make sense to me.

Has anyone seen this behaviour before? If So, please help. many thanks
Chris