Super Bowl ad sales have been slower leading up to this year's game on Fox.

Only 90 percent of Fox's ad slots were sold out as of early December, a benchmark historically hit in September or October.

Advertising executives blame increasing competition and risk, a heftier price tag, decreasing returns and NFL’s own woes.

The cost of a 30-second spot has doubled in just a few years, from $2.5 million in 2010 to $5.5 million today.