When I flush my toilet, the bowl empties as usual and fills as usual, but the water level in the bowl begins to slowly drain and maintains a lower lever than normal. I don't know what is causing this or how to correct the situation. This has been going on for approximately a week. I have checked the basement and don't find any leaks in the drain pipe. I don't know what else to check. Can you tell me where my problem is and how to fix it? The toilet does still work, but the water level in the bowl does not stay as high as it used to.