To say T-Mobile had a nice bounce-back year in 2014 would be a huge understatement. The “Un-carrier” not only added a net 8 million subscribers — including 4 million branded postpaid phone subscribers — but it also unleashed a bunch of new initiatives that gave customers more value for their money that rival carriers were quickly forced to adopt.

FROM EARLIER: T-Mobile went on an absolute rampage in 2014

However, the company still has one glaring weakness and it’s the same glaring weakness that it had at this time last year: The overall reach of its wireless network. Consumer Intelligence Research Partners has just sent us some data from its new survey of wireless subscribers and has found that the biggest reason customers leave T-Mobile is the quality of its network.

Source: CIRP

As you can see in the chart above, almost 40% of former T-Mobile and Sprint customers cited network quality as their main reason for switching carriers. In comparison, price was the biggest reason by far that Verizon and AT&T customers switched providers, as more than half of former Verizon and AT&T customers moved to another provider due to costs.

The tricky thing for T-Mobile is that there’s no quick fix here. While the “Un-carrier” has made strides in improving its mobile data network’s overall speed, it still has problems reaching areas outside of big cities, mostly because it lacks access to sufficient spectrum to reach many areas. And with the 600MHz auction getting postponed until next year, it may be a while before T-Mobile has the chance to acquire the spectrum it needs.

Prior to joining BGR as News Editor, Brad Reed spent five years covering the wireless industry for Network World. His first smartphone was a BlackBerry but he has since become a loyal Android user.