CHARLOTTE, N.C. — One of the mysteries about the Carolina Panthers' 1-3 start involves the play of what was supposed to be one of the NFL's best offensive lines.

All five starters returned this season, led by 2008 all-pro left tackle Jordan Gross, but so far the line has struggled both in run blocking and pass protection.

The Panthers rank 23rd in the NFL in rushing and have yet to get a 100-yard performance out of either of their highly-regarded backs, DeAngelo Williams or Jonathan Stewart.

According to www.stats.com, the offensive line has given up 10 sacks through the first four games. That's already half of the Panthers' team total for all of last season.

"We're a little slow right now, but it'll come," said right guard Keydrick Vincent. "We've just got to keep working on it."

But what's happened since last season, when the Panthers ranked third in the NFL in rushing and eighth in fewest sacks allowed?

"This is a new year," said left guard Travelle Wharton. "People attack you different."

The Philadelphia Eagles surprised the Panthers with an exotic concoction of blitzes in a 38-10 season-opening loss at Carolina. Opponents since then have used eight-man fronts to thwart the running game and have double-covered Steve Smith to slow the Panthers' receiving star—all tactics Carolina saw last season en route to a 12-4 record.

Coach John Fox is protective of the offensive line, saying Wednesday that he wasn't sure the unit's play should be singled out.

"We've had the same guys in there," he said. "It's probably (that) we need to improve.

"We do that in practice and hopefully we'll continue to get better."

Unlike on defense, injuries haven't been an issue hindering the offensive line's development. Its five starters have been healthy since the start of training camp.

There's plenty of talent on board. Gross and right tackle Jeff Otah were first-round picks, while center Ryan Kalil was a second-rounder. Wharton is frequently spoken of as having Pro Bowl potential at left guard.

Gross acknowledged he expected the line to pick up where it left off last season, especially since each of the five starters participated in the offseason conditioning program together.

"We've definitely been working hard," he said. "We've got a lot of reps together. It just hasn't seemed to click yet—running the ball, protection, everything."

Gross was particularly upset with his play during last week's 20-17 win against Washington, when he appeared to give up two sacks (one fewer than all of last season) and apologized to his teammates.