Imported picture for timelapse not complete

hi. I‘m trying to do a timelapse using PP. i followed the instruction i.e. select 1st image, click Image Sequence n Open.

After imported, PP only read some of picture and not the whole pictures in the folder.

My timelapse should be around 15-20secs. But it was cut in the middle and become around 3 secs.

Pls help!