Larger caliber, different shaped pellet or different shot placement 

That is interesting. You have plenty of gun and power so shot placement is your key now. Cannot tell you how many hundreds of pigeons I have shot with a low power HW55 springer. They almost never flew away. Head shots or chest shots in the heart/ lungs . Neck shots also work. Look up the anatomy of the pigeon and study the location of the vitals. Practice and you will get it. Go here: https://www.google.com/search?q=pigeon+internal+anatomy&sa=X&ved=0ahUKEwj7--m_k-7cAhXLrVQKHUpCCA8Q1QII9QEoAg&biw=1440&bih=662 



Check your pellets as you may be powering straight thru and not getting enough transfer of energy. The Huntsman is a great gun so at that close range if you cannot reduce the power then try using a wadcutter pellet . It does not take a lot of power to kill pigeons. Enjoy!
 
I had VERY similar experiences with .177 and .20 calibers powerful enough that I thought they'd way-lay pigeons just fine, but didn't! Conversely, I've dropped a couple other pigeons instantly with single .22 pellets at 450 FPS from a Crosman 150 pistol. 

Assuming good shot placement and adequate penetration, pellet WEIGHT is a non-issue. That leaves shot placement and caliber, or a combination of both. To quote from my first book (The Manic Compressive, copyright 1998)- 

"Crows and pigeons are tough, and in my experience, often able to shrug off reasonably good hits from a .177. However I find the same hits with a .22 will often drop them. I now try to keep .177 shots on crows very high in the chest, while .22 is somewhat more forgiving of mid-chest hits."

By the way, if you hit the spine or backbone, it doesn't matter what caliber; they're coming right down!
 
Its shot placement. You dont need fancy pellets. If you are shooting straight on at the base of the neck you are most likely hitting above any organs that would cause an incapacitating shot.This is even more of a factor if you are shooting up at the pigeon. If the shot is at an upward angle you are going completely over any vital organs going for a base of the neck shot. Pigeons heart and lungs sit kinda low in the chest so a shot at the base of the neck will most likely completely miss the heart and if you are lucky just clip the top of the lung. If you just clip the top of the lung the birds going to be able to fly off. Just like socaloldman said you should look at a pic of the pigeons organs and lower the shot more into the chest and the pigeons will start dropping.