If you dynamically generate any form controls that post data or cause a postback, you need to recreate them again on postback in order for them to be bound to their data, or their events, after the postback. Conceptually, this makes sense. If you don't have a control in your form after the postback, how could you look at its contents?
There are several ways you could approach this problem.
1) Call GenerateForm() no matter what. Since you said it pre-populates some of the data, you would need to change it so it can be called without doing that. ASP.NET will populate the controls with the data posted automatically on postback, which is what you want.
2) Keep a list of all your dynamically generated controls in a ViewState variable, so you can re-generate them upon postback. For most situations involving dynamically-created controls that aren't very simple (e.g., you may not know in advance exactly what controls are generated), this is the best solution. And often you will want to be able to access the data after a postback, but maybe you really don't want to recreate the whole form because you aren't using it any more.
As long as you recreate a control of the same type and ID on or before Page_Load(), it will be bound to the posted data. It does not need to be in exactly the same place on your form. And it does not need to be used or displayed, either - you can destroy it before the form is rendered, e.g., in Page_PreRender()
3) If you have no interest in any of this, you can always use Request.Form to look directly at the posted data, though this can also be tricky because the names will likely not match your form control IDs exactly. ASP.NET generates unique client-side IDs that depend on the container, and this is what you'll find in Request.Form. If you don't regenerate a control, you may not be able to easily determine the ID that you are looking for. Generally, you should not do this, but it's a way to look at the posted data and sometimes you need it.