I've been having some issues when rendering a lot of pixels at once (think large storage spot), but not a lot of the pixels are moving. Maybe, if a pixel and the neighbors haven't moved in ~100 ticks, the pixel could "freeze", and not be checked for movement, until at least 1 pixel within the check area has moved, but the area it blocked by walls. See the ASCII art below for an example. M is moving, || is a wall, and F is frozen.
Ex 1
M || F F F F
M || F F F F
M || F F F F
M || F F F F
Ex 2
M F F
M F F
M F F
M F F
Ex 2 next tick
M M M
M M M
M M M
M M M