Meaning: Put things in a pile
Example: I STACKED UP the boxes.
Example: Work STACKED UP while I was away on holiday.
Meaning: Increase, accumulate something
Example: I've been STACKING UP a lot of air miles.
Meaning: Be logical, make sense
Example: The budget figures don't STACK UP.
Meaning: Build up the number of planes waiting to land at an airport
Example: Planes were STACKING UP while the airport was closed after the bomb threat.