I've had this problem with several X-10 chimes. The problem is tuning. Open the chime and you well see a small square tunable coil. It is in a shiny aluminum can. There is a threaded tuning slug in the can that you can access from the top of the can. Be very careful of the wires that connect to the piezo sounder; they are soldered to this sounder but will not take much flexing.
Obviously you must have the chime plugged into your 120Volt house wiring to do this test with the CASE OPEN - MEANING 120 VOLTS are exposed and you could make contact with those exposed connections if you aren't careful or watchful of what you are doing.
What I do is set up a X-10 trigger program in the ISY that I can trigger with a push button like a mini remote. (You can't use the power unplugging method to do this; it must be a real X-10 signal.) MARK the position of the slot in the tuning slug with a fine felt pen on the aluminum can so you can get back to your original position if necessary. Using a NON-METALLIC tool (the slug is magnetic, you can't use a metallic screwdriver with a iron or steel blade. a metallic tool drastically affects the magnetic properties of the slug. using a metal blade will render this testing useless.), having a plastic blade that fits in the tuning slug slot, rotate the tuning slug by very small amounts; TESTING the chime with every small rotation of the slug. You may find the chime starts to work again. You will likely have to try rotating the slug in both clockwise and counterclockwise directions. If this doesn't fix the problem, the part of the chime that detects the X-10 signal may have failed.
In my case, I have found these chimes are quite sensitive to temperature, particularly the summer heat. They quit working when my house gets hot. Tuning them slightly has fixed the problem for me. It may take more than one turn of the slug. COUNT the TURNS!