If you make a traditional slime block player launcher to throw a player into the air, and you stand at the edge of the slime block then you are pushed into any blocks that are around you - causing you not to be launched.
The first clip in the vid is what happens if you stand on the edge of the slime block & the second clip is when you stand more or less in the middle (or not working vs working)

Edit: It's actually more broken than I first thought. It seems any block next to the players trajectory causes the player to get stuck.