Hi vB2023.
It sounds as if, by missing 5 consecutive questions in the middle of your test, you drove the difficulty of the questions you were seeing down. Then, you got some correct and others incorrect, keeping the difficulty at that relatively low level.
That situation, combined with the fact that you missed 15 questions, resulted in your scoring much lower on this test than you scored on your last one.
Do you know why you missed 5 in a row? Sometimes people miss multiple GMAT questions in a row because of a loss of focus, which sometimes occurs because they got thrown off by the first question in the group they missed.
If you can figure out why you missed 5 questions in a row and address the issue, you'll hopefully keep yourself from doing the same thing on test day.