pixel_player907117485 it is almost always due to stepping on the 2 when going from 1 to 3.
You have to move Diggy very methodically, step by step, in any puzzle in which his normal choice would be to walk over a number that is not in your unlocking sequence. You will encounter this issue also in puzzles that have penalty tiles, those tiles that if stepped on, erase progress or produce high-value penalty tiles. In such cases, click his path for him carefully and slowly.
