This has been driving me insane aswell for a LONG time. I hate to be the one who says it but just yesterday I received my wahoo speed and cadence sensor and installed it this morning. Has been working like a charm atleast for this ride (And I prefer not having that annoying arm flopping around).

As others have noted I think it is a question of the speed sensor (And specifically the SPEED sensor) not being triggered. When this happens the unit eventually shuts off and you dont receive anything. I have been guessing that this is due to the reed switches just being piss poor in those things. Good luck.