By Katie Marsal

Published: 11:00 AM EST

Apple Computer is once again facing a problem that it has become all too familiar with -- not enough supply of its most popular products to meet growing demand.

In particular, the Cupertino, Calif.-based company this month is reportedly struggling to fill orders for its consumer-oriented MacBook notebooks, which are arguably its most popular new product of the year.

Weighing on the Mac maker is the back-to-school shopping bonanza, a potentially lucrative period each year in which student buyers willingly plunk down chunks of cash for the latest computer systems before the start of fall classes.

This year, Apple's MacBooks are being billed as a hot ticket, but the company (or one of its component suppliers) has seemingly underestimated demand. Many customers who were expecting to receive previously-placed orders for the 13-inch Intel notebooks by this week got something else instead -- an apology letter from Apple stating that it would be unable to fill orders within the timeframe it had promised.

"I ordered a MacBook over the weekend," said one customer. "The order was to be processed in 5-8 days. I just got an apology letter from Apple saying they would not be able to meet that order fulfillment deadline."

It's unclear how far back the MacBook backlog stretches, but customers who custom-configured their systems with added memory and hard drive space are reporting delays up to two weeks from when they first placed their orders.

"I ordered a MacBook a week ago with a promise of shipping within 24 hours. The ship date was then set for yesterday (8/21)," another customer said. "I have now recieved notice that my computer won't ship until a week from today (8/29)."

The insufficient supply of MacBooks is not necessarily a new problem. During a recent conference call, Apple executives said that despite shipping as many units as it could during the spring, the company still exited its June quarter with backlog. However, they anticipated reaching a supply and demand balance on the notebooks by the end of September.

In general, sales of Apple's notebook computers have been on the up and up this year, with the company's share of the US notebook marketing doubling to 12 percent during the six-month period ending June. Though it did not fulfill all of its notebook orders during its most recent quarter, they company still shipped a record 800,000 units.

In an attempt to form a long-term solution to its notebook supply difficulties, the Mac maker has been scouring the far east for a third manufacturing partner to compliment current partners, Asustek and Quanta. With insiders believing Apple holds the potential to sell upwards of 1 million notebook systems during its December holiday quarter, the company will surely need the added help.