Sticking keys are generally as result of moisture build up in the keyboard. The structure of a piano keyboard is primarily made up of the wooden keys as well as a number of paper and cloth felts. Sticking notes develop when any or all of these three components attracts moisture build up.
In climates such as those found in South Australia and Western Australia, piano keyboards typically remain free for most of the year due to the dry hot climate found in these states. When the weather is unseasonally humid or if there are long periods of rainfall, moisture in the air can build up which reulsts in the wood, paper and felt components in piano keyboards to swell. This is why keys start to stick.
In most cases, the following basic steps can be taken to temporarily repair a sticking piano key:
1. Identify the sticking key.
2. Depress the keys either side by using the thumb and index finger.
3. Grab hold of the sticking key by using the pad of the thumb and index finger.
4. Gently wiggle the sticking key from side to side for around 5 seconds.
5. Release and test the key.
If this doesn't work, the problem may well be somewhere in the piano action which will require a qualified piano tuner / technician to repair.