I have a 1930s era upright piano that I play perhaps three times a week for about an hour at a time. I'm playing very simple bass / chord progressions to woodshed jazz/soul compositions for a dance band so no Rachmaninoff or otherwise heavy concert type use. The piano stays well in tune and is in a temperature and humidity controlled room of the house. Lately the D above middle C key stays depressed (ie. will not return to its un-played position after playing) about 1 in 10 times. It will stick 1 in 2 times if the sustain pedal is held down or I'm playing major 7ths or something with the keys on either side of it being simultaneously depressed. The other keys come back up, D sticks down. When I open up the lid I can see the hammer that strikes the D string in question returns but the white keyboard key sticks in the down position and has to be lifted up with very little effort. Is this a common ailment with a simple fix or should I consult a piano repair professional?
The felt hammers in a piano can strike several strings at once. With extended use, small grooves appear in the felt where it contacts the strings. In time, these grooves deepen to the extent that they will sometimes trap the hammer in the strings. A piano tuner will often use a small wire brush to spruce up the felt before deep grooves appear. As a conservative DIY measure, you might try opening up the piano, looking for grooves in the face of the felt, and levelling them out with your fingernail/fingertip.