No reaction to Levulan blue light therapy even though the Levulan was applied for 70 minutes. What could be the reason?

There are ways to optimize the effect of the levulan application. I have been doing Levulan application with activation by IPL for the last 14 years with 100% activation of the levulan application. Several factors are important. First the skin needs to be cleansed thourghly with acetone scrub with 4x4 gauze pads until no yellow color of the oil from the face is present on the gauze. Can take up to five minutes usually to adequately cleanse the face for optimal penetration of the levulan into the skin. Secondly, the levulan is a tube of powder inside a tube of solvent liquid that when crushed, needs to be shaken for a minimum of 3 full minutes to adequately mix the levulan. The last issue is how long to let the levulan incubate in the skin. The longer, the better the result. At my clinic we let incubate overnight, usually 14-17 hours. The most important thing about that is to keep the area to be treated covered from any ultraviolet light until the IPL is used in the office to activate the levulan. This is usually accomplished with skip mask, hat, and sunglasses. All that is assuming that the levulan stick to be used is not expired.
The second part of your question is about CO2. Unless it is the older CO2 which is 100% ablative requiring anesthesia and a long uncomfortable recovery, the newer fractionated or micro-fractionated CO2 lasers would not treat AK's 100% as the laser is fractionated and does not cover 100% of the treated area.