Last October, iSuppli published a forecast of 43.7 million iPad units to be sold in 2011, based on Apple’s sales figures for the September quarter, even though the 4.19 million iPads sold in that quarter was seen as disappointing by Wall Street.

The launch of the iPad2 resulted in a crushing demand, and Wall Street became optimistic again, but then Apple struggled to keep up with demand for the iPad 2 as the backlogs started to increase. The optimism of Wall Street analysts expected Apple to report sales of about 6.2 million iPads, but they were constrained by the supply of 4.69 million units.

iSuppli was optimistic for 2011 and forecast 43.7 million iPad sales, but has now reviewed its predictions and cut down the number to 39.7 million. The review comes after Apple published their quarterly report, and iSuppli notes that the new expectations are a result of Apple’s inability to meet demand, rather than as a result of diminished consumer interest in the iDevice.

Specifically, iSuppli quotes supply chain sources who indicated that Apple had quality concerns with LCD panels, production shortages of the new speaker found in the iPad 2, and lamination issues with one of its touch panel suppliers. And while Apple is said to be on track to “significantly increase its production volume” in the current quarter, it is said to be “still falling substantially short of its target production goal for April,”  AppleInsider noted.

However, for 2012 iSuppli has increased its forecast to 62.6 million from its previous estimate of 61.6 million.