Sigler, 28, and Ferrara, 30, met on the Entourage set in 2008 when Sigler joined the HBO comedy as a guest star – playing the on-screen love interest of Ferrara's character, Turtle. They soon became a couple in real life, too.
"They're busy with their careers, and she in particular has a couple of movies out in 2010, and others in the works for 2011, so it's just tough," a source who knows the couple tells PEOPLE.
"It's ironic, because in the recent season finale of Entourage, Jamie tells Turtle that she has to go to New Zealand to shoot a movie, and it causes problems, so it's ironic but that's what their lives are like."
Sigler is best known for playing Meadow Soprano on HBO's The Sopranos from 1999 until 2007. In 2003, she married her then-agent, A.J. DiScala. They split in 2005.
Most recently, she's been seen in the ABC sitcom Ugly Betty.