Pavel,

Try more frequent waterings while the plant is spiking until it finishes blooming. I've dramatically increased bloom times with more water. The plant needs it while it's trying to support all the super-structure of blossoms (essentially hydraulic pressure, and you have to keep the pressure up), and the flowers themselves. Normal water levels aren't sufficient, and many plants will terminate the blooms if there's insufficient water. More water = longer blooms (sometimes.) Plants will also set fewer spikes and buds if optimal water isn't availbable.

Don't rot them, but err on the side of more frequent watering and you should see more flowers that are longer lasting.

Julie