It sounds like one of the following:
1. Muck on the tone hole edge. (Some pads are made form substandared leather which seem,s to retain some corrosive material from the tanning process. This keeps corroding the tone hole edge, making sticky green verdigris.)
2. An issue with the spring being a bit weak.
3. The face of the tone hole is too wide and therefore has too much surface area, most often because it is burred to a "T" shape.
3. Some friction in the key's movement that should not be there, eg bent pivot tube or mis-aligned posts.
4. Most likely: A substandard pad that is inherently sticky. This occurs when the pad maker useds a leather waterproofing/airproofing treatment that is inherently sticky. If this is the case a valid but possibly temporary band-aid is to use "Yamaha Powder Papers". You can make your own, but not so good, by rubbing talcum powder (without cornflower!!!) or Teflon powder into note paper and shaking off all the residue. Best solution, install a high quality pad of a respected brand, eg Music Center, who knows how to do waterproofing without making them sticky. (What exactly is your tech using?)
It could be a mixture of several of the above.