What causes failure to shed?

[Annarose15]

Back in October, my ball python went into blue, but he never shed his skin. He had the same dull look until February when he went into blue, but it took him another two weeks for him to shed the skin. It appeared to be one layer of skin of normal thickness, but what would cause this?

The OP doesn't ask anything about shedding in pieces.

I can't speak to what happened in October, but it is the normal process for a snake to go "blue", clear up, and then actually shed a week or so later. They don't go straight from blue to shedding.

Add: If he shed fully in one or two pieces this time, there is no need to bump your humidity.