Oxygen is a radical scavenger. At the air-film interface of a curing UV coating, atmospheric oxygen reacts with the propagating radicals faster than they can add to acrylate double bonds, terminating the chain reaction in a thin surface layer while the bulk cures normally.
The result is a coating that measures cured by most tests but feels tacky, marks easily and has poor scratch resistance. It is the most common cure defect in UV processing.
There are four standard remedies: nitrogen inerting the cure zone (effective, expensive), increasing lamp intensity or photoinitiator loading (partial, costly), formulating with amine synergists (the usual commercial answer), or increasing film thickness so the inhibited layer is proportionally insignificant.
It is worse at low film thickness, low line speed, low lamp intensity and in LED UV systems.