Enter Key On UserForm To Next Control Not Working

I have a User Form that is used to collect data. CommandButton1 loads the User Form data into the worksheet and CommandButton2 clears all data from the Form in preparation for entering the next record. Immediately after a user first opens the Form, the very first time CommandButton1 is clicked, the Enter Key stops working. At this point data can be typed into any TextBox on the Form, but when the Enter Key is pressed the cursor remains in the TextBox. (The Enter Key will not move the focus to the next Textbox in the Tab Order. Also, if I alter the code to set the focus on a CommandButton as the active control instead of a TextBox, pressing Enter on the active CommandButton does not execute the CommandButton's macro... the same behavior as a TextBox; nothing at all happens when the Enter Key is pressed even though the CommandButton has focus).

At the point when the Enter Key stops functioning, if the user presses 'Alt-Tab' to leave the Form and then immediately uses 'Alt-Tab' to return back into the Form the Enter Key suddenly works again and continues to work correctly from that point on even after CommandButton1 is clicked. Again, the Enter Key stops working ONLY the first time CommandButton1 is run immediately following initially opening the workbook and Alt-Tabing into the Form immediately corrects it. I saw one other post in this forum with this same problem in 2003, but unfortunately it did not get answered.