The New York Times and Washington Post both discuss how a little-noticed provision in a federal spending bill last year is creating problems for the implementation of the ACA. (According to the Times, it has "has tangled up the Obama administration, sent tremors through health insurance markets and rattled confidence in the durability of President Obama’s signature health law.")
The provision limits payment to insurance companies for what are called "risk corridors," which (the Times explains)
were intended to help some insurance companies if they ended up with too many new sick people on their rolls and too little cash from premiums to cover their medical bills in the first three years under the health law. … The payments were supposed to help insurers cope with the risks they assumed when they decided to participate in the law’s new insurance marketplaces. … [W]ithout them, insurers say, many consumers will face higher premiums and may have to scramble for other coverage. Already, some insurers have shut down over the unexpected shortfall.