Hello Lynn,
I have tried the similar scenario and that worked as expected. Here am pasting my code and please make a double check with your code.
aspx:
setting the RadMenu z-index by overriding the default CSS:
or you can simply achieve the same by setting the z-index property of the RadComboBox to a value which should be greater that RadMenu default z-index value which is 7000.
Please take a look at this help article also.
Controlling absolute positioning with z-index
Thanks,
Shinu.