animation gets stuck on fly in and fly out

I have several problems with animations that I would be happy if you could halp me with:

1. I try to create a snow animation with an png image that enters in a flyin animation (from the top to the center of my scene) and goes out of the slide, in a exit animation. The enter animation lasts a few seconds on the Timeline, but after it does, the exit animation does not work, and it stayes in the center.

2. Another problem is that when I increase the animation duration from 0.75 to more than that (in order for the snow to enter more slowly) the image of the snow gets stuck. As I increase the gear, it gets stuck earlier respectively and does not reach its final end position.

I'd love to hear tips on how to create a smooth animation like the one i described :)

thank you

Valeria