If the decapping rod is tightened securely in the resizing die, and is still sliding up during the resizing - decapping process, it sounds as if the decapping pin is not long enough, and the decapping rod is contacting the case web, forcing it to move. By any chance are you decapping military brass which has crimped in primers? Crimped in primers can require more force to remove, but if the lock nut is tight, you should not experience any problem in that regard.
Put an unprimed resized case in the shell holder, and check to see if the decapping pin extends below the case head when the ram is at full travel. If it does, then the lock nut holding the decapping rod in the die is not tight enough, allowing the decapping rod to move.
If the decapping pin does not extend past the case head, check to see if you can adjust the decapping rod further into the die, or if you can thread the die closer to the shell holder without causing more shoulder set back than you need. If the decapping rod, and die are adjusted properly, and you cannot decap your fired cases, something is out of spec, and needs to be replaced.