Thank you for your feedback. There is a currently known issue that when a second round is started before all blocks have fallen from the screen that these may freeze up. We also put a warning regarding this problem up on the project page and will publish a fix once the GMTK GameJam 2021 voting ends!
The various materials may fall when there are no adjacent materials to the left or right. This is signaled by a slight color change from yellow to red to black before falling. These mechanics are also described on the project page in the „How To Play (Spoilers)“ section.
We apologize for the inconvenience and hope that you still enjoyed our game!