Why won't the oven door close fully in one go?

I am having a huge issue with my oven. The oven in question is a gas oven, Hygena Concorde Deluxe. I acquired this oven when I moved in with my mum. I use the oven a lot as I enjoy cooking and baking... but the oven door is an absolute nightmare!
Every time I try to cook or bake in this oven, my cooking in the oven always, without fail, involves me having to slam the door shut as it won't close without slamming. But today, the bleeding thing still wouldn't shut even when slammed shut, and I got really angry after three failed attempts. I seriously DO NOT want a repeat performance of this again - it may affect the end result of what I'm baking, not to mention continuous banging is inconsiderate to the neighbours! What can I do about it?