ComboBox_Change Event Not Triggering

I have attached the workbook in question. The problem lies on the Diameter combobox in the Materials tab. If Diameter is left as a dash, the regular items (Item…) should appear in the list. When a diameter is selected, the diameter items (Dia…) should appear. I have not yet experienced the problem prior to switching to a new multipage tab but it happens frequently when I have entered many items without closing and reopening the form. When the problem does occur, I switch the diameter again to a random selection and then back to the correct one and it works fine. Thank you in advance for any input you may have regarding the change event not triggering or anything else about the form you happen to notice as I am completely self-taught and could use a lot of improvement I'm sure.

PS: To avoid error messages while testing, enter a random number into the Job Number field preceded by a capital T. Click the “Start Entering” button on the Item List page to bring up the form.

Thank you!