WASHINGTON (AP) — Frustrated that cars recalled for serious safety defects aren’t getting fixed quickly enough, the nation’s auto safety chief hosted a gathering of automakers Tuesday to search for solutions.

In some recalls for problems as serious as air bags that can spew shrapnel into drivers or fuel tanks that can rupture in a rear-end crash, completion rates are below 15 percent, six months or more after the recalls were announced.

These recalls involve millions of vehicles, challenging automakers to find both replacement parts and the cars’ owners. Regulators at times have fined automakers for dragging their feet. Meanwhile, concerned car owners wait for repairs and worry about the safety of their cars.

So, National Highway Traffic Safety Administration chief Mark Rosekind is asking auto companies and safety advocates for solutions.