As I've discussed many times, changing your door gasket regularly will help to keep your autoclave working error-free. Most makes and models recommend annual replacement, although Midmark recommends semi-annual checks with changing "if required" and annually at a minimum for their M9 and M11 series of autoclaves. Thus, it's not unusual to change the gasket on a M9 or M11 after only 6 months.
Most door gaskets are made of flourosilicone and, like a good pair of jeans, are designed to shrink to fit. This means it may well be a little loose when first installed. So that the gasket fits well when it shrinks and to prevent warping during installation, there is a specific technique to follow when installing a door gasket. The order is similar to tightening the lug nuts on a car tire, or the lugs on a drum head. The diagram below indicates the order in which to "tuck" the door gasket in place.
If you just seat the top and run your finger around the edge, you'll likely get a big bulge in one corner. As the door gasket is expected to shrink a little, it's normal to have a little excess material. By using the order shown above, you'll more evenly distribute the excess to get a better fit and good seal.
As also mentioned in a previous post, a little soapy water to lubricate the door gasket will help everything seat properly too.