It’s normal for it to be tight especially when the spr8ngs are brand new. Over time, it’ll loosen up. The reason you see 17 on the witness holes is because the tube itself holds 17 and the base pad holds an additional two rounds.
Keep them loaded for a few day and it’ll be easier to fit 19 rounds. By the way, even though my shadow 2 mags can hold 23 rounds, I don’t load them to 23 during a match except for the first mag.