Had this on the Tandem with a triple and it was combination of two things. Originally it was 8 speed but I changed to 9. Narrower chain on the 9 speed and I did not change the front derailleur. I also changed the granny to a 24 from the 28 fitted. It was the chainring that was the problem. I fitted one out of the spares box and it was steel and quite thin. The chain was not being thrown far enough with the 8 spd derailleur and the thin chain ring allowed to chain to slip between granny and middle. I got a new set of matched chainrings to cure the gap between the rings but still had a problem going from middle to granny. New proper 9 spd derailleur and all cured.