One thing to remember. The cbo will not allow you to type in a value that is not in the rowsource list. I don't know if this is what is happening but if it is then you will need a way to add things to the list. I use rightclick().

Typically I populate a cbo like this...

Init

this.rowsource = "select field from table where field NOT LIKE ' ' "+;
                 "order by field into cursor mycursor"
this.requery()

with this.rowsourcetype set to 3.

Then if you need to add to the list, rightclick to expose a popup of some kind then do this.requery() again to refresh the cbo.