In LabQuest 2.8.4 or newer, be sure the Photogate Timing Mode: Projectile Launcher has the “Use Time of Flight Pad” option selected.

In older versions of the LabQuest App, make sure the experiment is set to end after 6 events.

The Logger Pro 3 experiment file located at: Experiments / Physics with Vernier / 08B Projectile Motion (Launcher)
This file is set up for using the Projectile Launcher alone. If you add the Time of Flight Pad to the experiment, this file will not automatically detect the change. Data collection will end after the ball leaves the barrel of the launcher (4 events). You can change the experiment to end after the ball strikes the pad by clicking the stop watch icon and setting the experiment to end after 6 events.

A more interesting file to use that also calculates the complete time of flight is located at:
Experiments / Probes & Sensors / Projectile Launcher / Time of Flight

Related Issues:
For Vernier Projectile Launcher (VPL)
The Time of Flight pad sometimes fails to work.

For Go Direct® Projectile Launcher (GDX-PL)
Go Direct Projectile Launcher Troubleshooting and FAQs