Another alternative is that you are applying pressure (even some small pressure) to it at an angle when you push it down. I was doing this a bit on my EEb tuba second valve until I settled on just the right hand and holding position. And if you have plastic valve guides, this can then have a lasting effect even after you stop applying any sidewards pressure. This is because your sideways pressure can result in raising a small "burr" on the plastic guide, which you then need to trim carefully. Or it's possible that the valve guide channel is a bit rough -- which in turn may roughen the guide, which will be worse if you are also applying any sidewards pressure.
I'm not suggesting that you start taking a knife or sandpaper to your guide or the channel, but it's something to look at and perhaps have checked out by a repair tech if you suspect that might be the trouble.
One thing you can do is pay very careful attention to how you are pushing that piston. And also, you can carefully and intentionally apply such pressure and see if you can repeatedly get the result of the sticking. If you can, then look more carefully to see if that's what you're doing as you play.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)