toilet ball with not stay full...please help

Hi,

Thanks in advance for any help.

My toilet bowl seems to be draining, which is causing the toilet tank to go down. Over a 8-10 hour time, the tank went down about 3/8 of an inch, and the water level in the bowl also went down.

Everything else seems fine. There is no water leaking anywhere (none visible).

Any help would be appreciated.

Thanks,