Is it a missed rep if you don't lock it out? Leave that question for the purists (SSBBT has a picture of a non-lockout: Phil Anderson, p. 139). Since the big obstacle in the deadlift is breaking the weight off the floor, rather than the lockout, I would increase the weight the next workout, even if one rep wasn't perfectly locked out.
This. Locking out is not the challenge. Getting the damn thing off the ground is the tough part.
As a newbie I don't speak from a position of authority, but I guess that if the weight is heavy enough to cause form to degenerate, one *might* not be quite ready for that weight yet (I must say I don't know if it is the case here, I merely speak in general terms), so I would favor not counting it as a rep. At the same time, when I look at my videos I see so many mistakes in my squat that I would have to say I rarely perform a perfect rep. Sometimes the wrist is in flexion, at other times the knee is shoved out enought, then the depth is not enough... By my own reasoning these should not count but I've increased the weight anyway. I suppose it is ultimately up to you and why you think you didn't achieve lockout.
In a competition, a DL that isn't locked out is a no lift (red light). Since we also want to maximize ROM for strength training, you should be locking out your DL there as well.
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Not really. This depends on the lifter's anthropometry and strength. Deadlifts aren't often missed at the start, they're missed higher up. You see missing off the floor when the lifter is losing grip, gives up early (common in new lifters), or gets too greedy with jumps at meets.
Roobo, you need to make up your mind to finish all your reps - stand all the way up - from here on out. The correction needs to happen during the workout though, so get someone to cue you on your sets.