My ultima 65 laminator is not sealing well on the left side. I've tried adjusting the upper tension which helps temporarily, but it reloosens after just a few minutes of use.

I fix these machines for a living. As the machines age the heaters in the red rollers dont heat evenly and cool areas dont seal well. If you have a heat gun check the temperature along both roller. Some variation is normal but very cool areas that dont seal well point to needing new heaters. GBC machines work well but the parts cost a fortune. if one heater is going bad the other one is too, change in pairs ONLY, heater sets are near 300 bucks last time I checked:(

By "upper tension" do you mean the hex screw located inside the left panel? If you tighten this until there is no space between the heat rollers it should eleminate the problem. If it is continues to loosen, back the screw out, apply a small amount of locktight to the threads before replacing the screw.You can find locktight at any hardware store usually in the Paint section near the glues. Hope this helps. Good luck