Thank you for contacting us. I am not quite sure I understand exactly what is the issue, so excuse me if my answer does not answer your question. Even though I tried I could not reproduce the case when the ComboBox is not displayed. For that reason, I would like to ask to open a new Support Ticket and attach a sample project that demonstrates the issue.
Please, note that a reason for your issue, might be the fact that you are not using the latest version of our controls. That is why we highly recommend you upgrade to Q2 Service Pack 1 which is coming this week.
I would like to make also a few remarks in order to avoid any possible misunderstanding. Please, note that the ComboBox in the cell will be displayed only if RadGridView is in edit mode. Further, the Winforms RadGridView supports only one editor to be opened at a time. This implies that there can be only one combobox visible at a time. In your situation it is the default behavior of RadGridView that does not allow displaying all RadComboBoxes.
Please, let us know if the issue persists after upgrading. I am looking forward to your reply.