Sometimes can't move mouse pointer to the top-left hot corner to access the activities withou click

Hello,
I am using Gnome 3.8.6 on Debain 11. There is a feature that I can move mouse pointer to the top-left corner to access the activities to access my windows and applications. But I find this function sometimes work sometimes not. That is sometimes I need to move my mouse pointer to the top-left corner and click the activities to access my windows and applications. What’s the reason? How can I tell whether I need to click the activities?
Thanks.

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