"Those 15 got lost because in the conference there was some procedural issue and Leahy didn't want to bother with it," said a Senate staffer.
"Bath salts were in the House bill. And they're not in this one. You'd have to ask Sen. Leahy why that happened," said a staffer for a House Republican.
Leahy did not respond to requests for comment from ABCNews.com, but a Judiciary Committee staffer defended Leahy's decision to put just two of the 17 substances in the final version of the bill, saying "Leahy's focus was to get done what the Senate started. The House bill was out there, but not in a formal way."
He argued that with a bitterly divided Congress, getting consensus on a bill as complex as the FDA Safety and Innovation Act was an accomplishment.
When asked why not criminalize drugs that the DEA says it needs listed to help keep the streets safe, the committee staffer said, "Sen. Leahy has been clear that scheduling controlled substances is not something to be taken lightly."
"It is not without implication to put a whole lot of chemicals on the federal drug schedule," he said. "It means putting more people in jail and makes it harder to seek legitimate uses for these drugs. Leahy is most comfortable sticking with what has been carefully considered."
On background DEA officials were frustrated that the bill did not go far enough, but publically the agency "commended House and Senate negotiators for agreeing on legislation to control 26 synthetic drugs."
The bill also gives the DEA new powers to temporarily declare drugs illegal without going through the lengthy scheduling process to permanently criminalize them. Under the new law the agency can place drugs on a two-year "emergency schedule." Currently, the DEA can only emergency schedule a drug for one year.
In the meantime, one DEA official said, agents will be playing a "game of whack-a-mole," discovering new drugs and trying to classify them fast enough to prosecute offenders.