There's a lot of different strategies you can employ when you hit a plateau. Try de-loading (e.g. 10% less weight) and working back up to the weight you plateaued at. Try adding drop sets after your main sets. Try implementing a periodization program. It could be related to your diet; perhaps you're not eating enough. It could be related to your sleep; perhaps you're not sleeping enough. It could be a result of supporting muscles that are lagging behind the main muscle utilized for a given lift, so do some research into supplemental exercises that could shore up those supporting muscles. Good luck!