General Motors ( GM) might not be able to regain the profitability power it has lost over the past two years, Fitch said Thursday, and as a result, the ratings agency lowered its opinion of some of the carmaker's debt.

Despite Fitch's belief that GM's truck business is a particularly competitive part of its overall product line, the agency downgraded the debt of the company and its financial services unit, General Motors Acceptance, to BBB+ from A-, with a negative rating outlook. Fitch also cited a "negative incentives environment," a share price that's down 27.3% year to date, and health care and pension cost concerns. The news follows a downgrade of the company's debt last week by Moody's.

Shares of GM were losing about 1.8% to $37.87 in early afternoon trading on the New York Stock Exchange.

Fitch believes GM might not be able to bring back the margins it has lost, notably because the company's car portfolio has been stagnant for several years. Further, "the industry has shortened its product cycle substantially over the last five years," Fitch said.

GM's niche products, such as the Hummer H2, which has done relatively well in the market, don't represent sizeable enough volumes to overcome the car portfolio's slack. But the Cadillac ESV and CTS cars counter that issue to some degree, Fitch said.

Longer term, Fitch has concerns about GM's competitive standing, especially in the U.S. and in light of the United Auto Workers negotiations. Additionally, earnings and cash flow have been dragged down by higher health care and pension costs in the last few years, and GM is the largest corporate provider of health care in the U.S. GM is expected to contribute up to $15 billion in pension money in the next five years.

On a positive note, GM has a strong liquidity position with $20.6 billion in cash, a better product mix in North America and strong lines, such as the H2, among others, the ratings service said.