I was making a redstone contraption and i had a piston pushing some slime blocks and i had a piece of obsidian right next to the slime blocks and it should not have stopped them but for some reason the game counts the obsidian as connected to the slime blocks and making it count to the piston push limit
The slime block will "stick" to most blocks to the sides of the slime block (leaves are one exception). If those blocks are immovable, or the result would be pushing more than 12 total blocks, the piston won't be able to extend.