I have brought mine up quite a bit but I still have some un-even development from right to left leg. I had a serious right leg injury years back that I think could be a culprit. I have been using an Iso Lateral Hammer strength standing hamstring ands its made a great difference lately.
Sounds like it's the injury. I have been fine with injuries (legs) but have dislocated my knee cap very badly. I started getting some knee pain after that.
My old gym had that machine and I loved it. Although my current gym has 2 standing leg curl machines and both are good. They are both very old but do the job. It's an old school gym full of a mixture of new and old. It also has 2 seated leg curl and 2 lying leg curl machines but out of the 4 only 1 is any good for me. They bought a new seated leg curl and I can't fit in it and it feels like an injury ready to happen when I try to.
My other gym which I am leaving soon has a cybex seated leg curl and it's the best I have ever used. I can destroy myself on it without worrying about my lower back. Equipment can definitely make a difference. I have started doing stiff leg deadlifts as well so that will help. My lower back can take them now (hopefully

). They have improved a lot so I should be happy but for the amount of abuse I have put them through you would expect a lot better

Gonna join my old gym again soon so I will be able to use that hammer strength standing ham curl machine again.