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 debris causing the issue so takes some patience 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).
The cleaning should basically be a buffing process. Can use regular blue Windex or Hope's Perfect Glass cleaner, also try Isopropyl Alcohol. Cheap paper towels (white ones that contain no fragrances, lotions, etc.) or Kimwipes is what we use for the cleaning. Just get them damp for the cleaning and then come back and buff it again with a dry towel/wipe to remove any cleaning agent residue. Remember the cylinder needs cleaning too if no luck doing just the piston. Might need to roll up a small but long piece tightly like a cigar to get it to fit into the cylinder and then can grab both ends to move it up and down withing the cylinder to clean it.
Here is also a video of cleaning a different piston from our public knowledge base but process is similar so perhaps it might also help:
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.
Only other option beyond that would be to get an RMA to send it in and have our Service group evaluate it to see if damaged or attempt cleaning themselves but usually this is not required.