interestingly I located the problem. It is quite amazing the naming convention for the html elements generation.
I thought the naming convention for all generated html component id is m1-myscreenname-mywidgetname. I discovered that for my combobox it is different. I walked through the html and what I got is m1-myscreename-hidden-select-mywidgetname. I used this and everything was fine. Will this be different naming convention for other widgets?
The next issue now is after dynamically populating the combobox, the selected item does not appear until I go select it from the list. Any solution please.
Thanks