When the water in a toilet bowl drops and the tank is full without leaks, it generally means you have a clogged vent. When water enters a pipe, it has to push ahead the air inside the pipe. After water goes through a vent, it replaces the water that was lost when the toilet is flushed.
When a vent is not working properly due to a blockage, the water will leave a vacuum causing the air from the outside to be drawn into the inside of the pipe. If the outside is blocked by water in the trap of the toilet, water will also be drawn in to replace the air. Until the air in the pipe is replaced by another vent or the water is able to run level in the pipe it will be able to then refill.
An Alexandria plumber would be happy to provide further assistance, if you are in need. For Alexandria plumbing service call 1-800-GET-ROTO and a licenses plumbing specialist will be sent out at your convenience.
You can also check out the Ask the Plumber section of our Web site if you have other inquiries about this or other plumbing problems and we will get back to you with more plumbing advice.