NZEC in Python code for DIGJUMPS.

Hello, I have this Python solution for Chef and Digit Jumps which is giving me an NZEC error. I believe the algorithm is correct and BFS-like, but I am new to debugging NZECs in Python. Can anyone point out what may be causing it? Any help would be greatly appreciated.

IDEOne link to my code

Also, any improvement suggestions are appreciated.