Usually there are a few different tuning bolts you can spin that’ll adjust the resistance and some other parameters. However, it’s also possible to blow out the internal mechanism by pulling the door shut faster than it wants to go. It’ll stop working right and sometimes leak oil. It’s possible that’s happened to this one, in which case the only answer is to get a new one, but that might not be a bad idea anyway, because this one has a bent rod.

You could try unattaching the rod (to see if it can be attached in a better location or something) and playing with the tuning bolts, but it might be worth just getting a new one.