Always funny to see that people who are into this kind of electronics (connecting modules) do not know how to trace the problems. For less than 50 buck you can find a small DSO oscilloscope with which you can measure signals, for about 100-150 bucks you can find a multi meter with a beter scope in one case.

Do not get me wrong. Hanging out on a large Dutch computer forum with also an electronics section I do see this almost every day. People stacking modules and programming some code but have no knowledge of electronics and so no real tools to trace issues if something goes wrong.