There are typically two trouble points in a deadlift: One is getting the barbell past the shins; the other is locking your hips at the top of the movement (otherwise referred to as the "lock-out"). This video — and these tips — from EliteFTS explain the problem, and what you can do. The most obvious reason you can't deadlift a certain amount of weight is that you're not yet strong enough, plain and simple. But aside from strength, there are a few other possible reasons:
- Your stance may be too narrow: In a conventional stance, your stance is generally about shoulder width, but sometimes it could be too narrow, especially if you can't use your glutes to complete the lift. To find out, stand with your typical width, stand up straight and intentionally squeeze your butt together — hard. If you feel like you can't, widen your stance by a few centimetres.
- Your "setup" is too haphazard: A good setup prepares both your body and mind for the movement. In particular, the article recommends this ideal sequence of events: Breathe in, brace abs, tighten up your back, get tension off the bar and drive your weight into your heels. Do all of this in a sort of fluid (not rushed) motion and pull.
- You're trying to make the lock-out a back movement: The deadlift is primarily you driving from the hips (a hip hinge). This is why you need to be able to squeeze your butt hard, as it's the hamstring, glutes, lats — the entire posterior chain — that help you complete the movement. As the article notes, think about "tucking in your tail" instead of arching or hyperextending your back.
These aren't the only reasons, but they're good starting points to look at if you're trying to improve — or even properly complete — your lift.
3 Reasons You Can't Lock Out Your Deadlift [EliteFTS]