It is impossible to solder aluminum with only tin, tin does not "wet" aluminum surface because of the instantaneous reaction aluminum/oxygen on the surface of the base metal (aluminum). I suppose the problem is that the tin will be in contact with aluminum oxide and not with the metal.
It is known (in this industry field) that an alloy of Tin/Zinc can solder aluminum metal, this alloy will "stick" to the aluminum surface, so what about the role (interaction/reaction)of Zinc in the "wetting" of the passivated aluminum ?
EDIT: aluminum can be soldered by tin/zinc alloys https://www.belmontmetals.com/popular-uses-for-tin-zinc-solders/
here a specific product https://www.belmontmetals.com/product/60-aluminum-solder-tinzinc/
I can say from my experience that you can solder aluminum even with a homemade alloy of tin/zinc 70/30 or 80/20 using a normal gas torch in free air.