Originally Posted by Gerry Hull

The leading edge of the pulley is too squared, and chamfered on the sides of the tooth rather than more towards the top corner that first meets the chain. Second, the thickness of the tooth where the roller of the chain rests in is not adequate. Sram lower pulleys are slightly thinner than the upper ones, which I'd guess is part of their 10,000 CAD effort (aeroglide!) to reduce the racket of their notoriously noisy drivetrains. The best fix is to take an upper pulley and chamfer the whole damn thing yourself, but that takes practice and those things can be expensive. A second, almost as good fix is to take the original bum pulley, round off the leading corner a bit and then go back and give it a sharper edge. 220 wet or dry sandpaper at first, then smooth with 400-600.

Is good to first see if it needs all this worrying about. Put bike on stand, pedal at speed and tweak the chain about a bit, bounce it from side to side. On my new Wifli it took nothing for the pulley to dechain and then the chain would get sucked up into the cage. No way would I have ridden it like that, not worth the risk. If it seems that this happens so easily it makes your gut a little uncomfortable, then a fix is appropriate.

the one in photo is a ceramic xx lower wheel, kept as a spare. Its a bit overdone but illustrative.

Chainkeeper design and positioning is best explained in a short video and I can't upload those here.