Why is my machine losing steaming power?

My machine is 15 years old and has been used daily with absolutely no issues. I regularly descale and am meticulous about priming before and after pulling shots and steaming. Recently my machine has lost the ability to maintain any pressure while steaming. The light comes on and within about 10 seconds there is nothing coming out of the wand and the light is still on. Looking over the Q&A on your site, I did not find anyone with the same type of problem. My trouble shooting efforts show the following: Water does flow perfectly during priming of the steam wand (with no leaks), the brew head and portafilter. The machine still makes excellent shots of espresso. The temperature of the water is hot. I simply cannot steam milk. Please help!