I was shooting a 887 today when it dropped a low shot like it was low on CO2. The next shot didn't make it out the barrel. So, I set it aside for a little bit, then removed the cylinder to see if it was empty. Felt pretty full, so I put it back in. Then it shot fine.
What was the temp? Sometimes if the temperature gets too high the liquid CO2 tries to become all gas & the resultanbt pressure will not let the hammer/spring operate the valve.
Or a dirty valve ... make sure there is no gunk in the valve area before you screw the cylinder in
when the shot is taken are you feeling any air coming out of the trigger assembly. There is a over pressure plug (looks like a head of a tac nail, black in color) I have seen these come dislodged and will casuse the pellets to fall low!!!
I am curious to learn more about this situation. We had a simular situation with an 888. It was brand new out of the box, first time at the range and it malfunctioned. I called the manufacture - they acted like this should have never happened and we have to return everything. We too followed all the measures listed in the manual, etc. and also tried a different cylinder. The 888 still would not function after this.