suspect that could be a css problem. The drop down may be visible but
perhaps not accessible because of the state of the surrounding div.
If the probelem were occuring in firefox, you could play around with
the css in firebug, but in IE it is less easy. Try making the div
containing the lookup position: relative; and see what happens. Or
for test purposes, move the autocomplete to another page element and
gradually work it back to where you want it.