Hi, Karen -
Generally, if something works in filler and not in browser, I suspect postback settings. So, the first thing I'd check is the postback on the drop down - right click and select properties. Then, under the browser tab, select Always for postback.
The other thing, I suppose, could be that your data source query is failing in browser, but I'd expect the form to put up a fuss over that....
Oh, a third thing - not sure what your filter is, but if it involves the user name function, sometimes I've seen browser return that in all lower case, while filler returns in upper case...
Hope that helps!