Why?
He could have been conserving ammunition. What was that term for ‘the simplest answer is often the correct one’? It didn’t matter.
Occam’s razor.
I’m not sure how well it applies here, though.
It was possible.
I moved my bugs closer to Mannequin, to test his presence for gas. Only a few perished.

Seems like an improvement.
There wasn’t much, if any. His mouth was closed. He was catching up to Grue. Grue must have noticed, because he directed Sirius up into an alley and towards a roof.
Hmm. Now he might be conserving ammunition – he probably has a limited amount of the gas.
Mannequin stopped and raised one arm, then fired. My bugs felt the concussion of the shot, but no reaction from Grue and Sirius.
There was a pause, then another shot. Again, no reaction. Two misses.