There are a number of things that can cause a key to stick. In this case, you might notice the hammer alignment. Notice that the first picture depicts very little to no clearance in relationship to the neighboring hammer. As the hammer travels towards the string the two hammers rub with one hammer unable to return to its normal position. This will cause a sticking key or a note that will not repeat. During a regular service call a technician can observe and correct this condition so that the player can then enjoy their play knowing that all the notes on the piano will perform properly.