That Russian resupply capsule crash from last week is causing more problems than just a lack of Tang for the astronauts on the ISS. If Russia doesn't figure out the problem and fix it faster than you can say "что идет вверх, должно снизиться," the station may get mothballed this winter.
In general, Russian rockets are quite reliable, mostly because their engines have stayed more or less the same for decades and all the bugs have been worked out. Well, most of the bugs. The same engines that were used on the vehicle that crashed last week are also used on the vehicles that carry people into space, so both the U.S. and Russia are understandably a bit wary of sending anyone up until they've figured out what exactly went wrong.
While the ISS can sustain its current crew indefinitely, the Russian return vehicles currently docked there are only rated to remain in space for 210 days each, and anything beyond that is in excess of their design life. This means that half of the six man crew has to come back by October, and the other three have to return to Earth by early January. Problem is, early January is a bad time to be landing in Kazakhstan, and to avoid classic Russian winter weather the last landing window is in mid-November. If a manned Soyuz mission doesn't make it to the ISS by then, the station will be empty for the first time in 11 years.
This whole situation exemplifies the problem of having only one way to get people to the ISS (and back), and the fact that it's an issue so soon after the decommissioning of the shuttle fleet and comes after Roskosmos proudly declared it to be the "age of Soyuz" just intensifies the irony. It's not that there's anything wrong with relying on the Russians specifically, it's more that just having one single reliable space transport system is just not reliable in the long run. Private industry is doing its level best to fill in the gaps, but until it does, we're just going to have to cross our fingers and hope that the Russians can make things happen in time to keep the ISS manned and operational.