My first thought would be mineral buildup inside the power head. In this case it would need a serious descaling.
Scale normally forms while the machine is being used; descaling is necessary every 4-6 months of use or whenever you observe a decrease in water flow. If you wish to descale the machine yourself, you may use any commercially available non-toxic and/or non-harmful descaling agent for coffee machines. Warning! Never use vinegar as a descaling agent.
Mix the descaling agent with water as directed on the package and pour the solution into the water container; place the water container inside the machine.
Turn on the machine, Position the steam tube over the drip tray. Empty the entire contents from the water container at intervals (one cup at a time), by turning the steam knob counterclockwise; to cut off flow turn the knob clockwise. During each interval, allow the descaling agent to act for about 10-15 minutes. When all the descaling agent has been used up, take out the water container, rinse it out and fill it with fresh drinking water. Place the container back inside the machine; empty out 2/3 of the water in the container by turning the steam knob. Let the machine heat up and empty out the water remaining in the container with the steam knob.
If you have been using it for a long time without doing this a couple times a year you may have to do it more than once to fully descale it.