Poor Sprint: it seems that the carrier just can’t seem to get a break. The latest Sprint problem has to do with the unexpected demand for the HTC-produced EVO 4G, which is the cornerstone in the company’s WiMax strategy.

While WiMax isn’t yet available in most markets, Sprint is making progress in getting more WiMax capacity available, but to take advantage of it you need a 4G WiMax smart phone; and this is where the EVO 4G comes in. The EVO 4G is a 3G/4G hybrid that allows use on both the traditional Sprint EVDO network as well as the WiMax 4G network when it is available.

The problem is that apparently HTC and Sprint didn’t think that the EVO 4G was going to be as much of a hit as it has been and lack of availability of the EVO 4G has become an issue. According to sources, parts shortages seem to be the biggest problem with producing enough units, and despite HTC signing additional agreements for secondary sources, apparently the handset maker still can’t get adequate parts to produce enough units to meet Sprint’s needs.

Estimates suggest that over 300,000 units have been sold so far, but analysts we have spoken with think that the number is behind 15% to 25% of where it could be if Sprint had enough units in stock. The demand for the EVO 4G is surprising given the limited availability of Sprint’s 4G WiMax network. From the shadows we hear that it could take HTC at least as long as another 45 to 60 days to clear back orders for the unit.