Mqdias:
I fully understand the problem, however I'm not sure that you understood the solution.
The problem: once you select "Air", it triggers a combobox change event that launches userform2. Now, the combobox says "Air" in it. If you go back to it later and choose "Air" again, the userform does not launch. This is because when the combobox says AIR and you choose AIR again, there is no change. If there is no change, it will not trigger the event.
The solution: input code to clear the combobox as soon as the userform triggers. It would be much easier to help you if you posted the actual code. Below is a small example:
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