Pull down the Resource List from the menu at the top left (the "Info Panel", which usually displays the tech you are researching). Under the Strategic Resources category, what does it say for aluminum? It may say something like "8 Aluminum 8 Used"
This most commonly happens when you got aluminum earlier in the game from AI trades or CS allies, built some aluminum-requiring buildings (Hydro Dams, Spaceship Factories) or units (Jet Fighters, Modern Armor, etc.) and later lost the aluminum, creating a strategic resource deficit. In those cases, your newly acquired aluminum is used first to eliminate the strategic resource deficit, so the newly acquired aluminum shows up as "used."
If that doesn't explain your problem, please post a save file. If no explanation jumps out of the save file, we'll move your report to the Bug Reports forum.