Please try to check following point and let us know the result in order to narrow down what may cause the issue.

1. Open WebComboSamples project (start > All Programs > Intersoft WebUI Studio 2009 R2 > WebUI Studio for ASP.NET > WebCombo 7 > C# Samples (or VB Samples)) and view “MultipleColumns.aspx” and “UnboundMultiCols.aspx” in your browser (in this case is IE7 browser). Does the issue persist?
2. Open http://live.intersoftpt.com/cs/WebCombo/MultipleColumns.aspx in IE7 browser. Does the issue persist?
3. Repeat step #1 and #2 with other browser such as Mozilla Firefox 3.5.5. Does the issue persist?
4. Repeat step #1 and #2 with IE7 browser from other computer. Does the issue persist?
5. Does it happen on single monitor mode or dual monitor mode?

Hope this help.

Ok, so the problem is the dual monitor.

The combo works perfectly on the primary monitor, but not on the secondary. Note that I only have the issue when I have a multicolumns combo.

Do you have a solution/workaround for that ?

found the solution ... set ResultBoxWindowType property to Normal

Floating result window has always been one of WebCombo’s key advantages. WebCombo’s result box can be “float” outside its main window. Unfortunately, this feature caused issue in dual monitor, i.e. the result box is not displayed in the secondary monitor. To solve this issue, you need to set ResultBoxWindowType property to Normal.

*This issue only occurs in IE.

Hope this helps.

source: http://www.intersoftpt.com/Community/WebCombo/Webcombo-dropdown-wont-show-on-some-IE7IE8-clients/

Glad to hear the good news.

Should you need further assistance or run into any problems regarding our controls, feel free to post them into our forum. We would be happy to assist you again.