maybe a lack of power, a short on circuit, a reversed LED, check every connection and check the schematics for any wrong connections.
oh i know this one, its the FRED photopopper.
Some troubleshooting tips on FRED engines there.
FWIW I'm a big fan of the balancing pot. I can never get matched FLEDs, photodiodes, whatever. I don't know if you need it or not in this case but they make it easier to keep poppers from spinning in circles when they are working.