I had a similar issue with a boulderdash style game I made once. What behaviours are you using for the blocks? Checking for overlapping at offset can cause issues, and there are several workarounds.
One possible way to do it is to have the blocks correct themselves when they stop falling. i.e. when overlapping at offset stop falling and then immediately move the block into the right position (which will be the Y of the block it is overlapping with + its height) in the same frame.
I can provide a short event block to illustrate this if you prefer.