Can't view a frame from a Live Photo

Greetings Miaanh,

Thanks for visiting the Apple Support Communities! I understand you're unable to change the key photo when editing a Live Photo, as you don't see the slider to change the frame.

To change the key photo, you'll need to tap the Live Photos button in the bottom-left corner after tapping the Edit button while viewing a Live Photo. You can view those steps in the following article: Take and edit Live Photos - Apple Support

"How to change the key photo

1. Open the Live Photo.
2. Tap Edit.
3. Tap the Live Photos button .
4. Move the slider to change the frame.
5. Release your finger, then tap Make Key Photo.
6. 
7. Tap Done."

Cheers.

I know I am taking Live Photos because the icon is still there, and if I tap the photo, it still plays like a Live Photo. Anyone else have this problem??

I had the same problem after updating to iOS 13.3. The Live Photos have still been playing, but the control to select the keyframe has been missing. When I clicked the "Edit" button, I saw that "Live" button has been crossed out in Edit mode. And when I viewed the Live Photos enlarged, the Live button has not been yellow, but gray.

It seems, we have to enable the Live button for each and every Live photo again, even if the Photos still seems to be alive, to bring the key frame slider back. It used to be different in the previous system versions. The Live button remained enabled, unless we turned it off.