Why is my Command-Control-Shift-4 screenshot not allowing me to adjust the range on MacBook Air

when I use command-control-shift-4 screenshot, I can drag on the part of screen I want to capture, and even my finger left trackpad, the screenshot mode still holding there for a second let me adjust the range quickly. however, right now it became take a screenshot when my finger left the trackpad immediately. please help me to fix this problem.

[Re-Titled by Moderator]