According to an article posted on the ESA website a couple of weeks ago, the performance of the spacecraft matched expectations but the performance of Venus didn't. Mission scientists were surprised by the variability of Venus' atmosphere. Venus Express may have detected atmospheric waves, and also saw a surprisingly rapid increase in atmospheric density from the night side to the day side of the planet.

Over time, Venus Express' orbit decays, and by last week the periapsis had got close to 200 kilometers and was falling very rapidly. To keep the mission going, Venus Express must perform periapsis raise maneuvers to keep the orbit from decaying right into the atmosphere. But the spacecraft is running on empty. ESA doesn't know exactly when Venus Express is going to run out of fuel, because it's actually a difficult problem to measure the amount of fuel remaining in a spacecraft fuel tank; most spacecraft fuel "gauges" are actually just bookkeeping of the amount of fuel used over the lifetime of a spacecraft. Small uncertainties in that bookkeeping add up over time, so it's hard to know just how empty the tank is. We're now within the error bars of ESA's estimate of where zero is. But it's not actually at zero yet, because the spacecraft is still successfully performing rocket burns.

The amount of fuel left in the tank is so low that Venus Express can't do its periodic periapsis raise maneuvers all in one burn anymore. Instead, mission controllers separate the burn into 10 or so pulses, each conducted at the orbit apoapsis. Venus Express is in the middle of one such series; it began on Sunday and continues all week. Each day, we could find out Venus Express just isn't there anymore. But it's more likely that engineers have been conservative in their fuel estimates, so there is likely enough fuel remaining to survive this week. The next periapsis raise maneuver is in February, and the subsequent one in June.

As long as Venus Express survives, it'll keep doing science. Venus Express science ground segment manager Don Merritt keeps us updated in the Venus Express thread at unmannedspaceflight.com: