It is a combination of several factors. You have a crank with 8 speed spacing between the rings along with a derailleur designed for an 8 speed shifter which will move the chain a sufficient amount to engage properly at that spacing. However, you are using a much narrower chain along with a shifter designed to move the chain a sufficient amount for narrower chainring spacing. Consequently the chain doesn't move far enough to engage the inner chainring especially when you are on the smallest cogs in back