Add a foaming agent like Delco's R-1400 Super Foaming Booster. They are designed to handle the exact problem you are running in to.
Delco Cleaning Systems
SUPER FOAMING BOOSTER is designed for enhancing detergent performance by increasing dwell time on vertical surfaces. This unique DELCO exclusive surfactant system boosts the cleaning performance of acid or alkaline based cleaning compounds in foamers.