A piston sticking is typically due to it being dirty. It can take several attempts to get it clean or find the one piece of small/invisible contamination causing the issue, so takes a lot of patience and a detailed process to help ensure get it clean. Make sure also cleaning the cylinder where it would come in contact with the piston if cleaning just the piston does not resolve the issue.
We recommend a very detailed process like cleaning the bench area where piston cleaning will be done thoroughly first, wash hands/arms prior, wear latex gloves, lay out clean wipes over area will be doing cleaning, and final step (which is very important) should be to blow off piston/cylinder with clean N2 or Air (do not use compressed air in a can like used for electronics cleaning) at a high flow rate.
The cleaning should basically be a buffing process. Can use the original blue Windex or Hope's Perfect Glass cleaner. Other similar cleaners have been shown to cause more harm than good so it is important to use one of these two specific cleaners. In some extreme cases could use alcohol but still recommended to use one of the above window cleaners after this. Cheap paper towels (white kitchen style that contain no fragrances, lotions, etc.) or Kimwipes is what we use for the cleaning. Just get them damp for the initial cleaning and then come back and buff it again with a dry towel/wipe to remove any cleaning agent residue. Using a lot of pressure while cleaning and buffing them often is the key to removing contaminants. Remember the cylinder needs cleaning too if no luck doing just the piston. Typically need to roll up the towel tightly to get it to fit into the cylinder and then can grab both ends to clean the cylinder area. A proven technique is to push the cylinder using the towel going through it so that it rolls on the bench while also sliding the towel left to right while it rolls to clean the inner surface. Be cautious as easy to break the towel and have the cylinder roll away.
Here is also a video of cleaning a different piston in another article. The actual cleaning process is the same so should also help:
Video: Cleaning the 10 kPa/kg Piston-Cylinder
With DWT's that use water as the media make sure it is only being use with Distilled Water. If it is not used regularly remove the piston and separate it from the system as have seen some cases where they corroded together or damage them if left sitting for long periods of time.