I don't have someone to help me in the gym, so I knew I had to record myself. My low back is rounded and it seems I can only get my upper back straight:
YouTube
The bar path does not look like vertical to the floor and I think I don't drop the weight fast enough.
Setup: I used 32.5kg (barbell+ 5kg and 1.25kg plates). That's a fairly low starting position and I hit my knees before I put 2x 10kg plates beneath the weights. If the barbell gets higher due to bigger plates in the future - will that help me straighten my lower back?
Sorry in case of bad english, greetings from Niedersachsen
Have you read the book? How much have you read about Starting Strenght and Rip? Have you seen any of the videos ?
If there's no problem setting your lumbar arch, why are you not doing it? Perhaps this will help: The Barbell Row | Mark Rippetoe