Transmission Oil and filter must be changed strictly as per manufacturer schedule and seals should be checked for leaks. Also the transmission oil level should always read full on the dipstick level or prescribed measuring reference.
Loss of Automatic transmission oil is always caused by leaks so if you have to replenish this fluid it is best to get it checked out ASAP. Use correct fluid and do not overfill as this can cause damage to the transmission.
If your automatic transmission is not regularly maintained then you risk serious failure, which may require a rebuild costing thousands of dollars.